fix(messaging, android): use only one FlutterLoader instance (#12845)

Author: russellwheatley

packages/firebase_messaging/firebase_messaging/android/src/main/java/io/flutter/plugins/firebase/messaging/FlutterFirebaseMessagingBackgroundExecutor.java

@@ -14,6 +14,7 @@
 import android.util.Log;
 import androidx.annotation.NonNull;
 import com.google.firebase.messaging.RemoteMessage;
+import io.flutter.FlutterInjector;
 import io.flutter.embedding.engine.FlutterEngine;
 import io.flutter.embedding.engine.FlutterShellArgs;
 import io.flutter.embedding.engine.dart.DartExecutor;
@@ -30,7 +31,6 @@
 import java.util.Map;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.atomic.AtomicBoolean;
-
 /**
  * An background execution abstraction which handles initializing a background isolate running a
  * callback dispatcher, used to invoke Dart callbacks while backgrounded.
@@ -149,7 +149,7 @@ public void startBackgroundIsolate(long callbackHandle, FlutterShellArgs shellAr
  return;
  }
 
- FlutterLoader loader = new FlutterLoader();
+ FlutterLoader loader = FlutterInjector.instance().flutterLoader();
  Handler mainHandler = new Handler(Looper.getMainLooper());
  Runnable myRunnable =
  () -> {