Add separate headset and speaker volumes

This is a simple implementation of keeping separate volume
settings for the wired headset and the speaker without
overhauling the audio system.

This allows a person with sensitive headphones to keep music, ringer
and other stream volumes lower than they would usually keep the
speaker volumes.

Change-Id: Ia614c1e3ac548ff4e0104a090cda13b9daef09cb