Wait for an abort event when stopping a model

Because the client callbacks from SoundTriggerMiddleware service are
async, if a client quickly stops and starts a model, then receives a
detection event for that model, it is impossible for it to tell
whether the event corresponding to the previous or current session,
and thus is unable to reason about the resulting state of the model.

To mitigate that, we will always wait for an abort event when a model
is stopped before any other operation is attempted. This way the
model state stays synchronized.

Bug: 191935600
Fixes: 226926627
Test: Manual verification of soundtrigger use-cases. Specifically,
      quick toggling of Assitant and Now Playing via Settings.
Change-Id: Ia1b91e78aa6981a7b3612ae9213af1b37a333d00