Fix thumbnail bug

When there was no thumbnail for a given image, the getThumbNail() convenience
method could return a previously-returned thumbnail instead of null.

b/15771860
https://code.google.com/p/android/issues/detail?id=40714

Change-Id: Ibd18e048145bf347469f800afdf436247ea6b693
Merge "Fix thumbnail bug" into pac-4.4