Conditional behavior for a11y stream volume

VolumePolicy: define two a11y modes: 1/ legacy where
  a11y volume aliases to media volume, 2/ a11y where
  the a11y volume is independent from any other stream
  type volume.
Refactor accessibility service listening to accomodate
  communicating to the volume controller when the
  a11y service is running.
Make accessibility stream alias conditional on a11y
  service running. Reflect the behavior in the
  volume controller.

Test: enable Talkback and verify logs in Vol controller
Bug 30448020

Change-Id: I80535ba259a22b6d93e62a7b3ca462a19d4c84b7