Rename and improve remote volume controller related methods

1. Renamed "RemoteVolumeController" to "RemoteSession" since it
better encapsulates the callback methods.
2. Added Javadoc for new parameters in
registerRemoteSessionCallback.
3. Renamed onSessionChanged to onDefaultRemoteSessionChanged and
revised documentation,
4. Added documentation for what the flags in
RemoteSessionCallback#onVolumeChanged are for.

Bug: 173657280
Test: N/A
Change-Id: I8f0d58f732df8117888bc66a58f47fc33db2339e