Fix overdraw issues in the conversation view

Remove the redundant window background and row backgrounds to improve
drawing performance.

res/layout/conversation_item_activity.xml

@@ -5,7 +5,6 @@
     android:layout_width="fill_parent"
     android:layout_height="wrap_content"
     android:orientation="vertical"
-    android:background="?conversation_background"
     android:padding="8dp">
 
     <TextView android:id="@+id/conversation_item_body"