Add first frame reporting to gallery's Play Store Console logging (#22579)

examples/flutter_gallery/android/app/src/main/java/io/flutter/demo/gallery/MainActivity.java

@@ -7,6 +7,7 @@ package io.flutter.demo.gallery;
 import android.os.Bundle;
 
 import io.flutter.app.FlutterActivity;
+import io.flutter.view.FlutterView;
 import io.flutter.plugins.GeneratedPluginRegistrant;
 
 public class MainActivity extends FlutterActivity {
@@ -23,5 +24,13 @@ public class MainActivity extends FlutterActivity {
         super.onCreate(savedInstanceState);
         GeneratedPluginRegistrant.registerWith(this);
         instrumentation = new FlutterGalleryInstrumentation(this.getFlutterView());
+        getFlutterView().addFirstFrameListener(new FlutterView.FirstFrameListener() {
+            @Override
+            public void onFirstFrame() {
+                // Report fully drawn time for Play Store Console.
+                MainActivity.this.reportFullyDrawn();
+                MainActivity.this.getFlutterView().removeFirstFrameListener(this);
+            }
+          });
     }
 }