Move public consts to HotwordRecognitionService

- The HotwordRecognitionService is the only public component of
  Hotword recognition allowing others to implement hotword recognition
- The client API is not available for use by all, hence the consts need
  to be moved to the service class.
- Ideally we'd have retained the public consts in the client side API
  (HotwordRecognizer)

Change-Id: Ibd0d1c0d0514ec4e8f7c284e22c5fa6b7ef3df26