Add flag and listener for a11y volume requests.

We're adding a dedicated volume level for accessibility. Services
can use the new flag to request that this volume be activated for
accessibility usages.

To let AudioManager know when the request state changes, adding a
hidden convenience method to check if any active service requests
the a11y volume stream. This method can be used to enable the
stream and to decide when to show the UI to change its volume.

AudioManager wanted a listener for changes to this flag, so rather
than add yet another special-purpose listener, I've added one that
gets called back whenever there are state changes in a11y services.
These changes happen infrequently enough that we shouldn't need
more targeted methods.

Bug: 30448020
Bug: 27899567

Test: Adding CTS in linked CL.
Change-Id: Ifc53314dc7d9a6ee3d50b04ebcc1a87280cafa5e