Gallery2: fix two same albums come out after double click tags

When user click group by tags in albumpage,it will start
albumsetpage to make gallery display the pictures which
are sorted by tags.Then albumpsetpage will new a thread
to load albums data.So two threads will be created if user
click group by tags twice quickly.So the data will be handled
by two thread.So the size of the albums will be add twice.
So two same albums came out.

For thread security,we add a synchronized to make sure the mAlbums can
be only handled by one thread.

CRs-Fixed: 562742

Change-Id: Ib1e2356fe25811c6d5d083cedd1861ce3bf7dea3