Fix example on web

Author: Ortes

example/lib/app/app.dart

@@ -304,7 +304,7 @@ class _ChewieDemoState extends State<ChewieDemo> {
                 ),
               ],
             ),
-            if (Platform.isAndroid)
+            if (Theme.of(context).platform == TargetPlatform.android)
               ListTile(
                 title: const Text("Delay"),
                 subtitle: DelaySlider(