[Gallery] Fix slow motion setting announcement (#301)

* Enable a11y carousel scrolling

* Fix slow motion announcement on android talkback

* Revert "Enable a11y carousel scrolling"

This reverts commit cc8f233b54.

gallery/gallery/lib/pages/settings_list_item.dart

@@ -26,7 +26,9 @@ class SlowMotionSetting extends StatelessWidget {
     final textTheme = Theme.of(context).textTheme;
     final options = GalleryOptions.of(context);
 
-    return Container(
+    return Semantics(
+      container: true,
+      child: Container(
         margin: settingItemHeaderMargin,
         child: Material(
           shape: RoundedRectangleBorder(borderRadius: settingItemBorderRadius),
@@ -66,6 +68,7 @@ class SlowMotionSetting extends StatelessWidget {
             ],
           ),
         ),
+      ),
     );
   }
 }