Fix 6341866 ANR in com.google.android.gallery3d due to...

Fix 6341866 ANR in com.google.android.gallery3d due to com.android.gallery3d.ui.TileImageViewAdapter.setScreenNail

RegionDecoder is run in a lower priority thread but it uses the same lock as setScreenNail while decoding, so a priority inversion causes this ANR.
The change tries to separate the lock so setScreenNail doesn't have to wait on the decoding process so the ANR could be avoided.

Change-Id: I02cc26fa0535adaa57cdcf94b819970e179311d1
b:6341866