enable Material 3 on android_splash_screen sample (#1604)

android_splash_screen/lib/main.dart

@@ -30,6 +30,7 @@ class MyApp extends StatelessWidget {
       title: 'Flutter Demo',
       theme: ThemeData(
         primarySwatch: Colors.blue,
+        useMaterial3: true,
       ),
       home: const MyHomePage(title: 'Flutter Demo Home Page'),
     );