Add view/drawable states for drag-accepting / drag-hovered

Added new drag_can_accept and drag_hovered XML attributes and the View
logic to support them.  Drawable states are now refreshed automatically
when a drag starts/ends and when a drag crosses the boundary of a
participating view.

Change-Id: I25f8ee02c83b3fa4f27201997d7eabf4be653fd8
5 files changed