audio policy: add support for virtualizer stage output

Add support for specialized output stream supporting virtualizer stage
effect with headtracking.

The HAL implementation indicates support for this feature with a
specific audio profile with flag AUDIO_OUTPUT_FLAG_VIRTUALIZER_STAGE
listed in the audio policy configuration file.

Note: the flag combination DEEP_BUFFER + FAST is used until a new audio
HAL version is introduced with this flag defined.

The audio policy manager implements methods to query if virtualization
is supported for a particular combination of audio attrbutes, audio
configuration and audio devices.
Methods are also available for the audio policy service to open and close
a specialized virtualization output stream and create the corresponding
audio mixer in audio flinger.

Bug: 188502620
Test: atest audiopolicy_tests

Change-Id: I3b1faf22290775dc7d13953a24098d22cb3790de