added Semantics wrapper over slider to add appropriate a11y semantic attributes to slider Before: https://screenshot.googleplex.com/8jmDh3RSB3oG5Z2 After: https://screenshot.googleplex.com/7Z4FdVgwcC4ZQxm fixes b/340638215
see [go/flutter-gar-reporting-q3-2023](http://go/flutter-gar-reporting-q3-2023)