Remote volume handling in MediaRouter

Extend MediaRouter.UserRouteInfo to enable setting playback
 information, which includes volume. When the user route instance
 has a RemoteControlClient, forward any playback information to it.
 Enable specifying a callback to be notified of volume events
 on the route.
Extend MediaRouter.RouteInfo to enable retrieving playback
 information.

Update RemoteControlClient javadoc to reflect which parts of the
 API are not intended to be made public.

Change-Id: I59d728eb61747af6c8c89d53f0faeb07940594c3