audio: implement mute on hdmi multichannel

On direct output streams the audio HAL must implement
the volume function. In the case of HDMI the only function
required is to mute audio when volume is 0 as volume
is defined as fixed on digital output streams.

Bug 8541062

Change-Id: I4b4e28a910e7b321b3a68567e9ad03fede065ca8
1 file changed