Send DRAG_STARTED notices to views that become visible during drag

We now recognize that a view is now a candidate for receiving the drop
but hasn't yet been notified about it, and send the DRAG_STARTED lifecycle
event properly.  This change also changes the DRAG_ENDED policy: now,
every view that had been sent a DRAG_STARTED receives the corresponding
DRAG_ENDED event, even though subsequent visibility changes may have
occured.

Change-Id: If4ae9ddbf414703e9f15eb2005471626bf19f316