Clean up the XR editor logic

- Coalesce common logic into the `main` flavor to avoid duplication
- Code cleanup

platform/android/java/editor/src/picoos/AndroidManifest.xml

@@ -29,15 +29,8 @@
 
         <activity
             android:name=".GodotXRGame"
-            android:configChanges="orientation|keyboardHidden|screenSize|smallestScreenSize|density|keyboard|navigation|screenLayout|uiMode"
-            android:process=":GodotXRGame"
-            android:launchMode="singleTask"
-            android:icon="@mipmap/ic_play_window"
-            android:label="@string/godot_game_activity_name"
             android:exported="false"
-            android:screenOrientation="landscape"
-            android:resizeableActivity="false"
-            android:theme="@android:style/Theme.Black.NoTitleBar.Fullscreen">
+            tools:node="merge">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />
                 <category android:name="android.intent.category.DEFAULT" />