Clear ref to animation after drag & drop animation completes

- LauncherAppTransitionManagerImpl -> DragLayer -> mDropAnim -> mDragView
  (via the update listener) -> mBitmap

Bug: 76162466
Test: Take hprof after picking up an app or widget, ensure there are no
      references to the image created for drag and drop

Change-Id: Iffc92eed13775921567f9ac25fea8e356d6fe963