Improve portability beyond Android: monoblend

See explanation at previous Change-Id I2a8db53bc4608fa8ab88602776104b6598da8337
except the monoblend portion is split out to permit soak testing before AOSP.

This is a private API that is only used at 3 places here:
 - frameworks/av/services/audioflinger/FastMixer.cpp
 - frameworks/av/services/audioflinger/Threads.cpp
and it appears the correct parameters are used.

Test: expect no change in behavior, but monitor for new failures
Change-Id: I6631890449e303268cec645dccf96b0466e52309
1 file changed