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
ac48767f91