Expose remote volume controller-related APIs

MediaSessionManager#registerRemoteVolumeController allows apps to
attach a binder to media service in order to be notified when
the session or volume of the remote volume controller is changed.

Since binders cannot be exposed, this CL instead adds and exposes
as system API a Callback interface that apps can implement.

Bug: 161332642
Test: Manual test by changing volume when remote casting
Change-Id: Ibb009559e818711ca40f7bc1b3694a49484960b3