msglist: Use faster fade to hidden animation for mark as read button

Fixes: zulip#613

Author: Khader-1

lib/widgets/message_list.dart

@@ -559,7 +559,7 @@ class MarkAsReadAnimation extends StatelessWidget {
   Widget build(BuildContext context) {
     return AnimatedOpacity(
       opacity: hidden ? 0 : loading ? 0.5 : 1,
-      duration: Duration(milliseconds: hidden ? 2000 : 300),
+      duration: const Duration(milliseconds: 500),
       curve: Curves.easeOut,
       child: child);
   }