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: I17ec0480cad4c3507a97454ccb832255adda9897
Merged-In: I17ec0480cad4c3507a97454ccb832255adda9897
Signed-off-by: Baekgyeong Kim <baek.kim@samsung.com>