Fix fullscreen scaling in AppRTCMobile.

webrtc/examples/androidapp/res/layout/activity_call.xml

 <?xml version="1.0" encoding="utf-8"?>
 
-<merge xmlns:android="http://schemas.android.com/apk/res/android">
+<!-- tools:ignore is needed because lint thinks this can be replaced with a merge. Replacing this
+     with a merge causes the fullscreen SurfaceView not to be centered. -->
+<FrameLayout
+    xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:tools="http://schemas.android.com/tools"
+    android:layout_width="match_parent"
+    android:layout_height="match_parent"
+    tools:ignore="MergeRootFrame">
 
     <org.webrtc.SurfaceViewRenderer
         android:id="@+id/fullscreen_video_view"
-        android:layout_width="match_parent"
-        android:layout_height="match_parent" />
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:layout_gravity="center" />
 
     <org.webrtc.SurfaceViewRenderer
         android:id="@+id/pip_video_view"
         android:layout_height="144dp"
         android:layout_width="wrap_content"
         android:layout_gravity="bottom|end"
         android:layout_margin="16dp"/>
 
     <FrameLayout
         android:id="@+id/call_fragment_container"
         android:layout_width="match_parent"
         android:layout_height="match_parent" />
     <FrameLayout
         android:id="@+id/hud_fragment_container"
         android:layout_width="match_parent"
         android:layout_height="match_parent" />
 
-</merge>
+</FrameLayout>