Fix CellLayout cast exception when drag is canceled

Test: Long press a widget and then release when the resize frame is
      shown. Then, click the content of the widget to launch the
      widget's activity. Go back to home screen and then immediately
      long press the same widget again. No crash was observed.
Bug: 189814379
Change-Id: I4232ecf9d9d9efaba817b7777314f0814bb7ef21