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.
https://github.com/LiquidSmooth/android_frameworks_base/commit/1fc952080133d8228d9ec49f57b542cb3b85b7cb

Change-Id: I06c3560463a1d9e7613353d1893cb19fa45e7f17