Gallery2: fix HDR crashes && fix exposure from being set.

* HDR transcoding is done on a different thread. got rid of the
stall in the transcooding step and possibliy fix race conditions.
* lock out the exposure step from being set, also fix an issue
of setting is being set by a method that is called on idle.

Change-Id: I9558558777a11cc8f6b633e397c809aa6993ceaf