This CL makes the system MediaRouter2 can receive transfer-related
events for the client application.
Also adds:
- startScan/stopScan
- setRouteVolume
- RouteCallback#onPreferredFeaturesChanged
- RoutingController methods' functionality
Also, this CL marks the system APIs as @SystemApi.
Bug: 181122201
Test: Passed tests locally. More test CLs are being added.
Change-Id: I0f733efb82faf2c4b0488a79335a97d4f7f7b466
ccb4075ff2