Always on hotword changes

Add model management API skeleton to VoiceInteractionManagerService
Add an "interactor" for all always-on APIs

- The VoiceInteractionService will get an interactor for the given
  keyphrase and locale.
- It can then check the availability and call methods to start and
  stop recognition on this interactor.

- Add a common class to deal with SoundTrigger APIs

- Cleanup the keyphrase representation:
  We now have separate representations for the keyphrase metadata and
  a keyphrase being used for recognition.
  This'll also help us to handle custom keyphrases in the
  future easily.
  This also ensures that for use within the framework,
  we rely on the ID of the KeyphraseInfo rather than comparing the
  text everytime.

Add a callback for the AlwaysOnHotwordDetector

This callback should be passed in by the VoiceInteractionService and is used to notify it
of recognition events.

Change-Id: I26252298773024f53a10cdd2af4404a4e6d74aae