Add more internal API's for assist

- When starting a voice interaction session from SysUI, a callback
can be passed to know when voice interaction has successfully started
- Add a new window type for the voice interaction starting window,
which resides behind the voice interaction layer.
- SystemUI now also inspects the voice interaction service to get the
logo asset for the starting window.
- Make VoiceInteractionSession window fullscreen, to accomodate for
the visuals.

Change-Id: If8c3c445e8b39841f48e8d153e6d1ba81e447286