Use RelativeLayout for the backing view

RelativeLayout allows the IME to place its UI element at an
arbitrary position on the backing view.

Change-Id: Ib40d2111f56d8f38007e37c6c47536db3f0bef75
diff --git a/java/res/layout/input_view.xml b/java/res/layout/input_view.xml
index ff0b403..7bfda3a 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. -->
-    <View
+    <RelativeLayout
         android:id="@+id/key_preview_backing"
         android:layout_width="match_parent"
         android:layout_height="0dp" />