MediaRouter: Call onDiscoveryPreferenceChanged()

After this CL, MediaRoute2ProviderService#onDiscoveryPreferenceChanged()
will be called when a callback is registered/unregistered from
MediaRouter2

To test the behavior, a new test class (MediaRoute2ProviderServiceTest)
is added as well.

Bug: 148771044
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

Change-Id: I9c5933f6074cc1cf41f524bd403e7ee842b4033c