Ensure drag outline & crosshairs disappear on drag cancel

Change-Id: I165d45846c0bfe74eda0d61b55e5b93fbfccdef2

src/com/android/launcher2/DragController.java

@@ -399,6 +399,7 @@ public class DragController {
      */
     public void cancelDrag() {
         if (mDragging) {
+            // Should we also be calling onDragExit() here?
             mDragSource.onDropCompleted(null, false);
         }
         endDrag();