System MediaRouter2: Add Transfer/ControllerCallback functionality

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