MediaRouter2: Add group volume

Add session volume or group volume info into RoutingSessionInfo.
Group volume can be controlled by MediaRouter2 or MediaRouter2Manager.

This CL includes minor clean up for CTS and MediaRouter2Manager.

Bug: 148994658
Bug: 149200358
Test: atest android.media.cts.MediaRoute2InfoTest
  && atest android.media.cts.MediaRouter2Test
  && atest android.media.cts.RouteDiscoveryPreferenceTest
  && atest android.media.cts.RoutingSessionInfoTest
  && atest android.media.cts.MediaRoute2ProviderServiceTest
  && atest mediaroutertest

Change-Id: If1d3aaae604a3c23d504e620ddb2bd2ffb260602