Eliminating the “black hole” effect with location/drop events.

Currently, a container view that doesn’t accept events, but has a
child that accepts events, prevents its parent from receiving
LOCATION/DROP events while the drag is over the container (but not
the child). This is a bug.

With this fix, such a container will prevent the parent from
invoking a (second) LOCATION/DROP event only if the event was really
delivered to any of its descendants.

To know whether the event was delivered, I added
DragEvent.mEventHandlerWasCalled member.

EXITED/ENTERED events are now generated upon delivery of the event
that has coordinates in it.

Current view that has drag focus is now global to reflect the fact
that it’s one per process.

Bug: 31469490
Change-Id: I248e8d1de87b7734853136eb4719f7571cea91d5