Fix MR2ServiceImpl nullability annotation

Test: Presubmit. No runtime changes. Only nullability annotation.
Bug: 205124386
Change-Id: I46ee33d934ed1b6594e607bf6278a50b38607654

services/core/java/com/android/server/media/MediaRouter2ServiceImpl.java

@@ -596,7 +596,7 @@ class MediaRouter2ServiceImpl {
 
     // Start of methods that implements operations for both MediaRouter2 and MediaRouter2Manager.
 
-    @NonNull
+    @Nullable
     public RoutingSessionInfo getSystemSessionInfo(
             @Nullable String packageName, boolean setDeviceRouteSelected) {
         final int uid = Binder.getCallingUid();