Fixing regression when removing last item from folder.

- The regression caused the folder and all its contents to be removed
  before the new item could be moved to the place of the folder.  In
  addition, in this case, the LauncherModel would get out of sync with 
  the db because we move the last item to the workspace, but don’t 
  remove it from the contents, so deleting the folder would remove the
  item’s mapping in sBgItemIdMap.

Bug: 24620815
Change-Id: I17137f28b0f1617a890488c7a9c5b8a9e8df0e91