Hide MediaSession details from MediaSessions clients

This will enable us to use MediaSessions for other streams that also
support volume adjustment (e.g. mirroring sessions), and not just media
sessions. We can later consider renaming the MediaSessions class to
something like VolumeSessions.

Bug: b/362507305
Test: Presubmit, build and verifying regular volume adjustment
Test: funcionality while casting.
Test: adb shell dumpsys activity service SystemUI | grep -A 100 "VolumeDialogControllerImpl state:"
Flag: EXEMPT Non-functional change
Change-Id: I51739d930d7cdb5b93c58394c699d4b6c3c8567c