Use FLUTTER_VIEW_ID to find the Flutter view (#87324)

dev/integration_tests/android_views/android/app/src/main/java/io/flutter/integration/androidviews/MainActivity.java

@@ -10,7 +10,6 @@ import android.os.Build;
 import android.os.Bundle;
 import android.view.MotionEvent;
 import android.view.View;
-import android.view.ViewGroup;
 
 import java.util.HashMap;
 
@@ -32,9 +31,7 @@ public class MainActivity extends FlutterActivity implements MethodChannel.Metho
     private MethodChannel.Result permissionResult;
 
     private View getFlutterView() {
-        // TODO(egarciad): Set an unique ID in FlutterView, so it's easier to look it up.
-        ViewGroup root = (ViewGroup)findViewById(android.R.id.content);
-        return ((ViewGroup)root.getChildAt(0)).getChildAt(0);
+      return findViewById(FLUTTER_VIEW_ID);
     }
 
     @Override

dev/integration_tests/hybrid_android_views/android/app/src/main/java/io/flutter/integration/androidviews/MainActivity.java

@@ -36,9 +36,7 @@ public class MainActivity extends FlutterActivity implements MethodChannel.Metho
     private MethodChannel.Result permissionResult;
 
     private View getFlutterView() {
-        // TODO(egarciad): Set an unique ID in FlutterView, so it's easier to look it up.
-        ViewGroup root = (ViewGroup)findViewById(android.R.id.content);
-        return ((ViewGroup)root.getChildAt(0)).getChildAt(0);
+      return findViewById(FLUTTER_VIEW_ID);
     }
 
     private String getViewName(View view) {