enable Material 3 on add_to_app/fullscreen (#1638)

add_to_app/fullscreen/flutter_module/lib/main.dart

@@ -64,6 +64,7 @@ class MyApp extends StatelessWidget {
   Widget build(BuildContext context) {
     return MaterialApp(
       title: 'Flutter Module Title',
+      theme: ThemeData.light(useMaterial3: true),
       routes: {
         '/': (context) => const FullScreenView(),
         '/mini': (context) => const Contents(),