Volume: we may need a service to manager the volume.

[feely]

Is your feature request related to a problem? Please describe.
We set the volume by property now, but it's hard to be managed.
I think the system needs a volume manager to manage all volume control requests from apps or key or intent, like a VolumeManagerService.

Describe the solution you'd like

Describe alternatives you've considered

[Yorkie Makoto]

Closing, because duplicated with #323.