Fix drag enter/exit reporting

Now, each ViewGroup is tracking which of its child views [which might
themselves be ViewGroups] is currently under the drag point, and when the
drag leaves that child, a DRAG_EXITED is synthesized and dispatched all
the way down to the leaf view previously under the point.  ENTERED is
still *not* dispatched down like this; instead, it's calculated and
synthesized directly at each level based on the new LOCATION.

The ViewRoot still tracks the leaf drag target, but solely for the
purpose of reporting changes to the OS after full dispatch of a new
LOCATION -- the entered/exited messaging is no longer initiated at the
ViewRoot level.

Change-Id: I0089cc538b7e33a0440187543fcfd2f8b12e197d