Simulates hardware gain control in audio driver

Change log
- audio_vbuffer in its own source and header files
- a pcm is shared via ext_pcm by multiple audio streams

Known issue
- Audio is distorted if multiple audio streams try to play
simultaneously. This is due to lack of proper audio mixing and will be
fixed in a future CL

Bug: 79496296
Test: manually

Change-Id: I14392e13ae49131a044efdda97ea23946287ad97