Merge "MediaRouter: save sessions in media router service"

media/java/android/media/IMediaRoute2ProviderClient.aidl

media/java/android/media/IMediaRouterService.aidl

media/java/android/media/MediaRoute2ProviderService.java

media/java/android/media/MediaRouter2Manager.java

...c/com/android/mediaroutertest/MediaRouterManagerTest.java

...re/java/com/android/server/media/MediaRoute2Provider.java

...va/com/android/server/media/MediaRoute2ProviderProxy.java

...ava/com/android/server/media/MediaRouter2ServiceImpl.java

...ore/java/com/android/server/media/MediaRouterService.java

...a/com/android/server/media/SystemMediaRoute2Provider.java