Volume control in MediaRouter dialogs

MediaRouter dialogs now intercept the volume keys for altering the
current volume. The status icon indicates if the slider/buttons are
currently controlling the local device volume or a remote device's
volume.

Group volume for user routes is handled by using the
RemoteControlClient supplied by the first route in the group.

Change-Id: I40a0d054847ed5acce7a4c3b669487841b4dca15