AudioMixer: fix artifact on 2 short sounds

On multiple devices, when 2 short sounds are played simultaneously, there will be artifact
on the speaker due to the buffer not being cleared.

A common example of this is lockscreen sounds during music playback.

Change-Id: Ifcdbbd7e443aa78ef392f14003d498404f2e4d3a
Merge "AudioMixer: fix artifact on 2 short sounds" into kitkat