Fix bug: moving items leaves workspace screens' caches in bad state

Change-Id: I88f9ded02b489bc189080afe75dc6f13af2cfcfd

src/com/android/launcher2/CellLayout.java

@@ -973,6 +973,7 @@ public class CellLayout extends ViewGroup implements Dimmable, VisibilityChanged
             }
         }
         prepareCacheBitmap();
+        invalidateCache();
     }
 
     @Override