Fixing crash in D&D due to race conditions during drag end.

*** MERGING TO MASTER ***

Drag-end event processing for a child view can remove the GroupView,
Which will synchronously call dispatchDetachedFromWindow(), which will
null mChildrenInterestedInDrag.
This causes a crash when trying to clear the map.

Fixing by introducing a local variable.

Bug: 25433279
Change-Id: I2ef88f7f97935dbafda54634831fbbff747b8f2e
(cherry picked from commit 2e2f1066ad89110365cdb504bf6568569d94da58)