Set the current/max volume of system routes

Also, this CL makes SystemMediaRoute2Provider publish new route infos
when the volumes are changed.

MediaRouter2 can get the volume changes from
RouteCallback#onRoutesChanged().

Bug: 149005682
Test: atest mediaroutertest and CTS
Change-Id: Icfc3b9f3b568e66d55782b11e0d29fec3b9eac6e