you can achieve it using frame layout only,
MainActivity should be extended as Activity, not UnityPlayerActivity. Add these two lines to activity in the AndroidManifest.xml.

<meta-data android_name="unityplayer.UnityActivity" android_value="true" />
<meta-data android_name="unityplayer.ForwardNativeEventsToDalvik" android_value="false" />

The working code is:

main.xml

    <?xml version="1.0" encoding="utf-8"?>  
    <RelativeLayout xmlns_android="http://schemas.android.com/apk/res/android"  
        android_layout_width="match_parent" android_layout_height="match_parent"  
        android_rowCount="3"  
        android_columnCount="10">  

        <FrameLayout  
            android_layout_width="match_parent"  
            android_layout_height="250dip"  
            android_id="@+id/fl_forUnity"  
            android_background="#ff00ff15"  
            android_layout_column="4"  
            android_layout_row="0"  
            android_layout_alignParentTop="true"  
            android_layout_centerHorizontal="true">  

        </FrameLayout>  

        <Button  
            android_layout_width="wrap_content"  
            android_layout_height="wrap_content"  
            android_text="Rotate Right"  
            android_id="@+id/bt_rotRight"  
            android_layout_column="4"  
            android_layout_row="2"  
            android_layout_alignTop="@+id/bt_stop"  
            android_layout_toRightOf="@+id/bt_stop"  
            android_layout_toEndOf="@+id/bt_stop"  
            android_layout_marginLeft="30dp" />  

        <Button  
            android_layout_width="wrap_content"  
            android_layout_height="wrap_content"  
            android_text="Rotate Left"  
            android_id="@+id/bt_rotLeft"  
            android_layout_column="9"  
            android_layout_row="2"  
            android_layout_alignTop="@+id/bt_stop"  
            android_layout_toLeftOf="@+id/bt_stop"  
            android_layout_toStartOf="@+id/bt_stop"  
            android_layout_marginRight="30dp" />  

        <Button  
            android_layout_width="wrap_content"  
            android_layout_height="wrap_content"  
            android_text="Stop"  
            android_id="@+id/bt_stop"  
            android_layout_below="@+id/fl_forUnity"  
            android_layout_centerHorizontal="true"  
            android_layout_margin="10dip" />  

    </RelativeLayout>  

UnityPlayerActivity.java

    import android.widget.FrameLayout;  
    import android.widget.Button;  

    public class UnityPlayerActivity extends Activity  
    {  
        protected UnityPlayer mUnityPlayer; // don't change the name of this variable; referenced from native code  

        //Declare a FrameLayout object  
        FrameLayout fl_forUnity;  

        //Declare the buttons  
        Button bt_rotLeft;  
        Button bt_rotRight;  
        Button bt_stop;  

        // Setup activity layout  
        @Override protected void onCreate (Bundle savedInstanceState)  
        {  
            requestWindowFeature(Window.FEATURE_NO_TITLE);  
            super.onCreate(savedInstanceState);  

            getWindow().setFormat(PixelFormat.RGBX_8888); // <--- This makes xperia play happy  

            mUnityPlayer = new UnityPlayer(this);  
            if (mUnityPlayer.getSettings ().getBoolean ("hide_status_bar", true))  
            {  
                setTheme(android.R.style.Theme_NoTitleBar_Fullscreen);  
                getWindow ().setFlags (WindowManager.LayoutParams.FLAG_FULLSCREEN,  
                        WindowManager.LayoutParams.FLAG_FULLSCREEN);  
            }  

            //setContentView(mUnityPlayer);  
            //Set the content to main  
            setContentView(R.layout.main);  

            //Inflate the frame layout from XML  
            this.fl_forUnity = (FrameLayout)findViewById(R.id.fl_forUnity);  

            //Add the mUnityPlayer view to the FrameLayout, and set it to fill all the area available  
            this.fl_forUnity.addView(mUnityPlayer.getView(),  
                    FrameLayout.LayoutParams.MATCH_PARENT, FrameLayout.LayoutParams.MATCH_PARENT);  

            //Initialize the buttons from the XML file  
            this.bt_rotLeft = (Button) findViewById(R.id.bt_rotLeft);  
            this.bt_rotRight = (Button) findViewById(R.id.bt_rotRight);  
            this.bt_stop = (Button) findViewById(R.id.bt_stop);  

            /*Send a brodcast a message to the 'Main Camera' game object, passing L, R according to the 
            pressed buttons.*/  
            this.bt_rotLeft.setOnClickListener(new View.OnClickListener() {  
                @Override  
                public void onClick(View v) {  
                    UnityPlayer.UnitySendMessage("Main Camera", "ReceiveRotDir", "L");  
                }  
            });  

            this.bt_rotRight.setOnClickListener(new View.OnClickListener() {  
                @Override  
                public void onClick(View v) {  
                    UnityPlayer.UnitySendMessage("Main Camera", "ReceiveRotDir", "R");  
                }  
            });  

            this.bt_stop.setOnClickListener(new View.OnClickListener() {  
                @Override  
                public void onClick(View v) {  
                    UnityPlayer.UnitySendMessage("Main Camera", "ReceiveRotDir", " ");  
                }  
            });  

            mUnityPlayer.requestFocus();  
        }  

I hope the below reference URLs might help you.
Unity and Android CustomLayout and Unity and Android sub-view

I’m using Unity with Android and it works when the view is in full screen, however when I make it into a subview, the screen doesn’t display anything.

It’s working as full screen:

Intent intent = new Intent(this, UnityPlayerActivity.class);
intent.putExtra("arguments", "data from android");
startActivity(intent);

These questions are also unanswered:

I’ve tried the solutions they have, but none seem to have worked.

Display Unity Scene as Sub View in android studio

https://gamedev.stackexchange.com/questions/134347/android-black-screen-with-unity-app-in-subview

Black screen unity subview in android

My current MainActivity/ Unity Manifest:

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        m_UnityPlayer = new UnityPlayer(this);
        int glesMode = m_UnityPlayer.getSettings().getInt("gles_mode", 1);
        boolean trueColor8888 = false;
        m_UnityPlayer.init(glesMode, trueColor8888);

        setContentView(R.layout.activity_main);

        FrameLayout layout = (FrameLayout) findViewById(R.id.frameLayout2);
        LayoutParams lp = new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT);
        layout.addView(m_UnityPlayer.getView(), 0, lp);
    }

<manifest xmlns_android="http://schemas.android.com/apk/res/android" package="com.unity3d.player" xmlns_tools="http://schemas.android.com/tools">
  <application>
    <activity android_name="com.unity3d.player.UnityPlayerActivity" android_theme="@style/UnityThemeSelector" android_screenOrientation="landscape" android_launchMode="singleTask" android_configChanges="mcc|mnc|locale|touchscreen|keyboard|keyboardHidden|navigation|orientation|screenLayout|uiMode|screenSize|smallestScreenSize|fontScale|layoutDirection|density" android_hardwareAccelerated="false">
      <intent-filter>
        <action android_name="android.intent.action.MAIN" />
        <category android_name="android.intent.category.LAUNCHER" />
      </intent-filter>
      <meta-data android_name="unityplayer.UnityActivity" android_value="true" />
      <meta-data android_name="android.notch_support" android_value="true" />
    </activity>
    <meta-data android_name="unity.splash-mode" android_value="0" />
    <meta-data android_name="unity.splash-enable" android_value="True" />
    <meta-data android_name="notch.config" android_value="portrait|landscape" />
    <meta-data android_name="unity.build-id" android_value="46e01524-60a4-48fa-9d28-b231f5db3c5d" />
  </application>
  <uses-feature android_glEsVersion="0x00030000" />
  <uses-feature android_name="android.hardware.vulkan.version" android_required="false" />
  <uses-feature android_name="android.hardware.touchscreen" android_required="false" />
  <uses-feature android_name="android.hardware.touchscreen.multitouch" android_required="false" />
  <uses-feature android_name="android.hardware.touchscreen.multitouch.distinct" android_required="false" />
</manifest>