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
a53b7051a2