Minor FakeSoundTriggerHal fixes

- Make sure setResourceContention is acked even when it is a no-op
- Make sure we don't send a null recognitionSession on stopRecognition,
  which is possible since the call is idempotent.

Bug: 236826280
Fixes: 280379313
Test: SoundTriggerManagerTest
Test: AlwaysOnHotwordDetectorTest
Change-Id: Ia62e6937f1a011aee0e37faa0cff8a0fd2bcbcac