Make hotword availability a callback

This helps us make the list sound models operation an async one, it also helps us
with the case where a detector is invalidated, so the client doesn't have to keep checking the
state.

Synchronize DatabaseHelper methods on its instance so that other VoiceInteractionManagerService
calls aren't blocked on db writes/reads.
It's still possible for the list operation to be blocked on update and vice-versa

Change-Id: Ib8ec4ac5056b62d443038560ce31d0641b4627b0