android: Fix background color within inset areas

src/android/app/src/main/res/layout-w600dp/activity_main.xml

@@ -5,7 +5,8 @@
     xmlns:tools="http://schemas.android.com/tools"
     android:id="@+id/coordinator_main"
     android:layout_width="match_parent"
-    android:layout_height="match_parent">
+    android:layout_height="match_parent"
+    android:background="?attr/colorSurface">
 
     <androidx.fragment.app.FragmentContainerView
         android:id="@+id/fragment_container"

src/android/app/src/main/res/layout/activity_main.xml

@@ -5,7 +5,8 @@
     xmlns:tools="http://schemas.android.com/tools"
     android:id="@+id/coordinator_main"
     android:layout_width="match_parent"
-    android:layout_height="match_parent">
+    android:layout_height="match_parent"
+    android:background="?attr/colorSurface">
 
     <androidx.fragment.app.FragmentContainerView
         android:id="@+id/fragment_container"