Do not block for HandlerThread start

Instead of calling HandlerThread#getLooper immediately after starting
it, do it asynchronously in onLooperPrepared.

getLooper blocks until the looper is ready, which may take a significant
amount of time.

This involves queueing tasks until the looper is ready, so the thread
selection logic is refactored to a dedicated class.

Test: atest
Change-Id: I0444bffe8e4bb4e3f29bd0271c7b24bc46a62cf6