Add audio stream and usage for virtual assistant

New stream type: AUDIO_STREAM_ASSISTANT
This is intended to be used by a virtual assistant like
Google Assistant, Bixby, etc.

The audio stream has own volume alias and the volume
does not change by volume changes of other streams.

Bug: 123745215
Test: make
Change-Id: Iad07d9f0b25ea35c6d565be677af7815bf2f6bd7