Change AppSearchManagerService's thread pool to use LinkedBlockingQueue.

If the service reaches the max pool size which is the number of
available processor, we should use LinkedBlockingQueue instead of
SynchronousQueue to queue the requests.

Bug: 185294190
Test: presubmit
Change-Id: I3a9a2f8a149f44fac7ddc01d61a6d1827a8d55bf