Clean up MediaRouter2Manager

This CL removes unnecessary APIs.

Bug: 155306249
Test: atest android.media.cts.MediaRoute2InfoTest
  android.media.cts.MediaRouter2Test
  android.media.cts.RouteDiscoveryPreferenceTest
  android.media.cts.RoutingSessionInfoTest
  android.media.cts.MediaRoute2ProviderServiceTest
  mediaroutertest;
  make -j42 RunSettingsLibRoboTests
Change-Id: I7ee6546deeaca8f5e9cb9b0d119ab6fff734ca1c