MediaRouter2: Implement RouteSessionController#release

This CL implements followings:
 - RouteSessionController#release
 - Router side logic of calling SessionCallback#onSessionReleased()

The test should be added when MediaRouterService actually notifies
the clients of session release. (i.e. No new test added.)

Bug: 146400872
Test: atest mediaroutertest
Change-Id: I2bad73e00986903b8c925704b0144d9e75f3bbf7
diff --git a/services/core/java/com/android/server/media/MediaRoute2Provider.java b/services/core/java/com/android/server/media/MediaRoute2Provider.java
index f11b70e..115155c 100644
--- a/services/core/java/com/android/server/media/MediaRoute2Provider.java
+++ b/services/core/java/com/android/server/media/MediaRoute2Provider.java
@@ -108,5 +108,8 @@
         // TODO: Remove this when MediaRouter2ServiceImpl notifies clients of session changes.
         void onSessionInfoChanged(@NonNull MediaRoute2Provider provider,
                 @NonNull RouteSessionInfo sessionInfo);
+        // TODO: Call this when service actually notifies of session release.
+        void onSessionReleased(@NonNull MediaRoute2Provider provider,
+                @NonNull RouteSessionInfo sessionInfo);
     }
 }