DO NOT MERGE: Don't crash when we get a drag-ended after being detached

Removing view groups during a drag would cause an NPE here.  The
child views will still get the drag-ended event as expected.

(Cherrypicked from downstream)

Bug 8298439

Change-Id: Ic14cbf9fdc305b91c1627f6882a45d34ee1c34ae
1 file changed