MediaRouter: Add MediaRoute2Info#isSystemRoute

MediaRoute2Info#isSystemRoute returns true only if
it is generated by SystemMediaRoute2Provider.

In addition, MediaRouter2 and MediaRouter2Manager
always include system routes when getRoutes or getAvailableRoutes
is called.

This CL will change the behavior of MediaRouter2#getRoutes(), however,
without this change, there doesn't seem to be a good way to get
system routes. (e.g. how can we support multiple bt routes in xMR?)

Bug: 145632124
Test: atest android.media.cts.MediaRoute2InfoTest
  && atest android.media.cts.MediaRouter2Test
  && atest android.media.cts.RouteDiscoveryPreferenceTest
  && atest android.media.cts.RoutingSessionInfoTest
  && atest mediaroutertest
Change-Id: I18ea188b68e6e1335cb0e25c3cc3674910d4d645