Add Various Accessibility Properties

[hmh84]

My company was testing our mobile app for accessibility with VoiceOver, three issues we noticed that are seemingly unsolvable in the package's current state/available properties were

1. When readOnlyHeadings={true} the headings do say "dimmed" which is fine however they should really be identified as a "header" as that's what it's trying to achieve instead of being read aloud like they are still selectable items.
2. Selectable items aren't read as "selected" by the screen reader and the user can't know what selections they have made. By adding accessibilityState={{ selected: itemSelected }} the screen reader now says "Selected, [Item name]" when an item is selected.
3. The select component that's rendered is trying to act as a combobox, and therefore the role should be combobox.

Please consider my changes, thank you for taking the time to review this pull request.

[github-actions]

This PR is stale because it has been open 45 days with no activity.

[renrizzolo]

Thanks for the PR!
I added the menuitem role to the sub item and the expanded state to the dropdown toggle.
@hmh84 I'm not able to test this so if you can confirm all OK, I'll merge

[hmh84]

Thanks for the PR! I added the menuitem role to the sub item and the expanded state to the dropdown toggle. @hmh84 I'm not able to test this so if you can confirm all OK, I'll merge

This works! Thanks