Use FrameLayout for the backing view

If we render our UI around the composing text in the fullscreen
mode, probably we need to have our views under
android.R.id.extractArea, which is implemented as FrameLayout.
Therefore, using FrameLayout for the backing view makes it easy
to simplify the code between normal mode and full screen mode.

Change-Id: I69533b22d9d1368ad906dc5ef7be1a1b674d10b8
diff --git a/java/res/layout/input_view.xml b/java/res/layout/input_view.xml
index 7bfda3a..a4bcdcc 100644
--- a/java/res/layout/input_view.xml
+++ b/java/res/layout/input_view.xml
@@ -25,7 +25,7 @@
     android:gravity="bottom|center_horizontal"
     android:orientation="vertical" >
     <!-- The height of key_preview_backing view will automatically be determined by code. -->
-    <RelativeLayout
+    <FrameLayout
         android:id="@+id/key_preview_backing"
         android:layout_width="match_parent"
         android:layout_height="0dp" />