audioflinger: add virtualizer stage mixer thread

Add specialized type of MixerThread for when
a global virtualizer stage with headtracking is used on a
HAL output stream.
This mixer thread has the following requirements:
- Implement a multichannel mixer but a stereo output stream
- Have an output stage effect chain with either a virtualizer stage
effect or plain downmixer effect and possbily other post processing effects
placed after the virtualizer/downmixer
- Being able to handle relatively short audio HAL buffers (5 to 10 ms)
while allowing audio effects to be applied (not a FastMixer).

Bug: 188502620
Test: make

Change-Id: I4dd5b4cd7da7ae0d6562a9a319bdb06b2e31e4e4