More drag/drop adjustments:

* Make View.onDragEvent() public instead of protected.
* No longer @hide View.startDrag()
* Properly manage the boundaries of DRAG_STARTED / DRAG_ENDED notifications
  to windows (and as a result don't send bogus empty DRAG_STARTED events or
  double-recycle pooled DragEvents)

Change-Id: Ib75f5c1417640c82a5b043c555e02d6e6f4b4d9c