MediaRouter2: Enhance readability of MediaRouter2ServiceImpl

This CL introduces following changes:
 - Renamed methods by adding 'Router2' or 'Manager',
   in order to easily distinguish the type of the caller.
 - Rename and reordered MediaRouter2ServiceImpl methods
 - Add missing @NonNull/@Nullable

Bug: 149642195
Test: CTS and atest mediaroutertest
Change-Id: I82758c1bb54e70b699e06fa46fa3a97462e41bc1

services/core/java/com/android/server/media/MediaRoute2ProviderWatcher.java

@@ -16,6 +16,7 @@
 
 package com.android.server.media;
 
+import android.annotation.NonNull;
 import android.content.BroadcastReceiver;
 import android.content.ComponentName;
 import android.content.Context;
@@ -171,7 +172,7 @@ final class MediaRoute2ProviderWatcher {
     };
 
     public interface Callback {
-        void onAddProviderService(MediaRoute2ProviderServiceProxy proxy);
-        void onRemoveProviderService(MediaRoute2ProviderServiceProxy proxy);
+        void onAddProviderService(@NonNull MediaRoute2ProviderServiceProxy proxy);
+        void onRemoveProviderService(@NonNull MediaRoute2ProviderServiceProxy proxy);
     }
 }