AlwaysOnHotwordDetector needs to reflect enrollment changes

Add a callback for when any sound model change happens. This helps the VIS
to re-check the availability and either enroll the user, or start/stop recognition.

Also shut down any active recognition when VIS dies, or a different hotword detector instance is obtained from VIS.

Change-Id: I03f94e78c6ee307afe822a84aebc7e74c64de7b4