...

public class MainActivity extends FlutterActivity {
  private static final String CHANNEL = "flutter.native";

  @Override
  protected void onCreate(Bundle savedInstanceState) {
...

    new MethodChannel(getFlutterView(), CHANNEL).setMethodCallHandler(
    new MethodChannel.MethodCallHandler() {
      @Override
      public void onMethodCall(MethodCall call, MethodChannel.Result result) {
        if (call.method.equals("moveTaskToBack")) {
          moveTaskToBack(true);
          result.success("OK");
        }
      }
    });
...