Add mic-only muting and playback volume control to MixingAudioDeviceModule

When screen audio is being shared in a 1-to-1 call, muting the
microphone should not kill the system audio stream. Added
setMicrophoneMuted() which zeroes out mic samples at the ADM level
while keeping the audio channel unmuted for system audio to flow.

Added setPlaybackVolume() to scale incoming playback audio samples
in NeedMorePlayData, enabling local volume control of the remote
user's audio independently of system volume.

Author: paterkleomenis