Add broadcast and query API for AudioSource.HOTWORD.

- When the AudioSource.HOTWORD input becomes active or is released, send
  a Broadcast with the package name and the new state of the audio input
  to any applications that hold CAPTURE_AUDIO_OUTPUT.
- Store the package name of the application that controls the hOTWORD
  input or set it to null if the input is not in use.
- Add a new method to AudioService to retrieve the package name of the
  application that currently controls the HOTWORD input.

Change-Id: I2f11888f3711d23b6287a4de7b81d361734a8f3b