Make all Android scenario_app activities full-screen, even on older…

… Android versions. (flutter#50666)

Work towards flutter/flutter#143471.

testing/scenario_app/android/app/src/main/java/dev/flutter/scenarios/TestableFlutterActivity.java

@@ -4,7 +4,10 @@
 
 package dev.flutter.scenarios;
 
+import android.os.Bundle;
+import android.view.WindowManager;
 import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
 import io.flutter.embedding.android.FlutterActivity;
 import io.flutter.embedding.engine.FlutterEngine;
 import java.util.concurrent.atomic.AtomicBoolean;
@@ -22,6 +25,20 @@ public void configureFlutterEngine(@NonNull FlutterEngine flutterEngine) {
         .setMessageHandler("take_screenshot", (byteBuffer, binaryReply) -> notifyFlutterRendered());
   }
 
+  @Override
+  protected void onCreate(@Nullable Bundle savedInstanceState) {
+    super.onCreate(savedInstanceState);
+
+    // On newer versions of Android, this is the default. Because these tests are being used to take
+    // screenshots on Skia Gold, we don't want any of the System UI to show up, even for older API
+    // versions (i.e. 28).
+    //
+    // See also:
+    // https://github.com/flutter/engine/blob/a9081cce1f0dd730577a36ee1ca6d7af5cdc5a9b/shell/platform/android/io/flutter/embedding/android/FlutterView.java#L696
+    // https://github.com/flutter/flutter/issues/143471
+    getWindow().addFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN);
+  }
+
   protected void notifyFlutterRendered() {
     synchronized (flutterUiRenderedLock) {
       isScenarioReady.set(true);