MediaSession: Add volumeControlId in MediaCotnroller.PlaybackInfo

This CL adds API to get/set volume control ID for a session.

Volume control ID is set by volume providers (e.g. MediaRouter)
to notify which volume provider is used by the session
to other apps such as System UI, Settings.

Bug: 147528363
Test: atest CtsMediaTestCases:android.media.cts.MediaSessionTest

Change-Id: I9ecafeca8b5090b4c8a89029d1e6411fd48919cb