Add stream/volume apis to sessions

This adds RemoteVolumeProviders which handle volume change
events and a way to switch a session between local stream
playback and remote playback handling.

This also adds a way for an app to specify the PendingIntent
to launch when we want to show their ongoing playback UI.

Change-Id: I3a72bf2ec7ca55f61f50859ddc2988eebd491e9d