Tweak the HotwordRecognizer API

- Make it more intuitive by enforcing callers to set a listener while
  starting recognition
- Fix TODO to resolve the ComponentName if none is provided
- Unbind from the Hotword service if stopRecognition is called
- Get rid of the callback in onStopHotwordRecognition

Change-Id: I8edad0ef0f3671283aeb51eaed1d3b8dab01baa0