Gallery2: Photo Gallery widget: fix NPE in getViewAt

Repro:
 - Add Photo Gallery widget to a home screen
 - Select "Shuffle all images"
 - Immediately drag widget to "Remove"
 - Observe NPE

The remote view is destroyed and the reference to mSource is null.
By making onDestroy and getViewAt synchronized methods, we ensure that
destroy cannot be executed to null out mSource while getViewAt is
being called.

Change-Id: I79466a11b528b1b1af0650927b7054837b631878