Enable Material 3 on `federated_plugin` (#1635)

* Enable Material 3 on federated_plugin

* update tests

experimental/federated_plugin/federated_plugin/example/lib/main.dart

@@ -14,8 +14,9 @@ class MyApp extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return const MaterialApp(
-      home: HomePage(),
+    return MaterialApp(
+      theme: ThemeData.light(useMaterial3: true),
+      home: const HomePage(),
     );
   }
 }
@@ -49,7 +50,7 @@ class _HomePageState extends State<HomePage> {
                     style: Theme.of(context).textTheme.headlineSmall,
                   ),
             const SizedBox(height: 16),
-            ElevatedButton(
+            FilledButton(
               onPressed: () async {
                 try {
                   final result = await getBatteryLevel();

experimental/federated_plugin/federated_plugin/example/test/widget_test.dart

@@ -22,7 +22,7 @@ void main() {
       await tester.pumpWidget(const MyApp());
 
       // Tap button to retrieve current battery level from platform.
-      await tester.tap(find.byType(ElevatedButton));
+      await tester.tap(find.byType(FilledButton));
       await tester.pumpAndSettle();
 
       expect(find.text('Battery Level: $batteryLevel'), findsOneWidget);