AssetAtlasService: fix a potential race condition

Backported from marshmallow-release.

Currently worker threads in computeBestConfiguration may NOT completely
finish before timeout.  But the code will start using the result while
the worker threads are still working on the same object.  This may
cause exceptions.

b/19966623

Change-Id: I62ffcb796d648891ee339b6a978f575ebad8352b