Fix A11y volume persisting/restore

When the accessibility volume is enabled, its value wasn't persisted
  when changed because it was still using the persisting name from
  its alias (music).
When enabled, notiify the volume controller that a11y volume has
  changed so the UI also reflects the new value, now updated from
  the persisted settings.
Do not persist volumes for streams that don't have a setting name.

Test: enable Talkback, set a11y volume at a different level than media \
  then disable Talkback. Reboot and enable Talkback, verify a11y is \
  restored.
Bug: 36286073

Change-Id: Ic6c30364e164b856fc10fbf6b22c09a7b5561be1