Guard access to SparseArray

SparseArray is not thread safe, and the bg dispatchers are
multi-threaded. Guard access in the following way:

* Use a double check lock when possibly creating it and returning it for
  tileSpecs
* Use a single lock and possibly return null if retrieving for
  operations (this is the same behavior as we had before).

Test: manual, tiles work
Test: manual, multi-user
Test: atest TileSpecSettingsRepositoryTest
Flag: EXEMPT bugfix
Fixes: 413289323
Change-Id: I6de31a59701f1e07b18fbc575b897134446444eb