audio: provide a hook for audio amplifier implementations

Example: http://review.cyanogenmod.org/#/c/38221

Change-Id: Ib236598a5888b2af19bcfb81e285f644a0e84c0d
Merge "audio: provide a hook for audio amplifier implementations" into cm-10.1
Merge remote-tracking branch 'upstream/cm-10.1' into cm-10.1