Media: add MediaRouter2.getRoutes()

As with this CL, media apps can get the list of available routes using
MediaRouter2.getRoutes() or MediaRouter2.Callback.onRouteListChanged().

For that MediaRouterService notifies MediaRouter2 of providerinfos.

This CL also clarifies provider info notification logic.

1) When a new client or a new manager is registered, it will be
notified.
2) When a provider info is updated, it will be notified.

onRouteListChanged will be called for a newly registerd callback.

Fixed a bug that MediaRoute2ProviderProxy didn't report state update
when the provider is disonnected.

Test: atest mediaroutertest
Change-Id: I50f5c3cabce80e7fb0a1b5596883c35911d6f122