How to create a channel from Android Service to Flutter function?

Question

I'm trying to create a channel between Flutter and Android.

I have this code inside MainActivity and this channel just works fine when called from FLutter code.

setMethodCallHandler { call, result ->
            Log.d("configureFlutterEngine", "${call.method} $result $call")
            if (call.method == "getBatteryLevel") {
                val batteryLevel = getBatteryLevel()

                if (batteryLevel != -1) {
                    result.success(batteryLevel)
                } else {
                    result.error("UNAVAILABLE", "Battery level not available.", null)
                }
            } else if (call.method == "startTrackingService") {
                Log.d("startTrackingService", "calling foregroundService()")
                foregroundService()
                result.success(true)
            } else if (call.method == "stopTrackingService") {
                Log.d("stopTrackingService", "calling stopTrackingService()")
                stopTrackingService()
                result.success(true)
            } else {
                result.notImplemented()
            }
        }

But it is not working the other way i.e. Native to FLutter. I have this code in My android service onCreate().

FlutterLoader().apply {
        startInitialization(context)
        ensureInitializationComplete(context, arrayOfNulls(0))

        val engine = FlutterEngine(context.applicationContext)
        val entrypoint = DartEntrypoint.createDefault()

        engine.dartExecutor.executeDartEntrypoint(entrypoint)
        methodChannel = MethodChannel(engine.dartExecutor.binaryMessenger, "android-flutter")

And this is called when everytime I get the location.

        methodChannel?.invokeMethod("getTracking", position.toJson(), object: MethodChannel.Result {
            override fun success(result: Any?) {
                Log.d("success", result.toString())
            }

            override fun error(errorCode: String?, errorMessage: String?, errorDetails: Any?) {
                Log.d("error", errorDetails.toString())
            }

            override fun notImplemented() {
                Log.d("notImplemented", "notImplemented".toString())
            }

        })
    }

And I have this in Flutter Widget.

void initState() {
  super.initState();
  MethodChannel("android-flutter").setMethodCallHandler((call) async {
    Fimber.i("$call invoked");
    if (call.method == "getTracking") {
      if (call.arguments is String) {
        Fimber.i(call.arguments);
        Position position = Position.fromJsonString(call.arguments);
        Fimber.i(position.toString());
      }
    } else {
      throw MissingPluginException();
    }
    return true;
  });
}

The invokeMethod(...) is getting called inside the service, but the method definition in flutter is not getting called.. After everytime invokeMethod() in service is called, The overridden notImplemented() method under the Result(){} is getting called.