Fix a couple of drag & drop crashes

1. ViewGroups being removed from the layout during a drag would wind up
crashing the app with an NPE at drag-ended time, due to blind dereference
of now-cleared object pointers.

2. Passing a 'null' ClipData to startDrag() would crash the system
process with an NPE.  Should this even be valid?  I'm inclined to say
yes, though it means that apps will need to guard against it.

Fixes bug 3369542

Change-Id: I168fc1284d6fd4403999946609725414cf254df0