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

Change-Id: I88f9ded02b489bc189080afe75dc6f13af2cfcfd
diff --git a/src/com/android/launcher2/CellLayout.java b/src/com/android/launcher2/CellLayout.java
index 73e0c8f..19e775b 100644
--- a/src/com/android/launcher2/CellLayout.java
+++ b/src/com/android/launcher2/CellLayout.java
@@ -973,6 +973,7 @@
             }
         }
         prepareCacheBitmap();
+        invalidateCache();
     }
 
     @Override