Mark cells as unoccupied in onDragStart instead of startDrag.

Also move cleanup (resetting variables to null) to onDragEnd
instead of onDropCompleted. These changes are necessary because
pre-drags (for apps with shortcuts) don't call onDragStart
or onDropCompleted.

Bug: 32246571
Change-Id: Ib18fac64555e9158b776f9c12afc2cb807b3c355
2 files changed