Complete implementation of menus and toolbars on mobile

### What is the problem or limitation you are having?

We currently have a partial implementation of menus and toolbars on Android, and no implementation of menus on iOS.

### Describe the solution you'd like

We should complete the implementation of menus on Android, and implement menus on iOS.

On both platforms, the content of the App menu should be accessed via the icon in the top-left of the navigation bar.

On Android, this means adding a top-left icon, and migrating the default About menu item to the App group.

On iOS, menus can be implemented with UIMenu.

### Describe alternatives you've considered

-

### Additional context

See discussions on #4271 for background.

The current implementation of the app menu (or the implemenation used by the default Scaffold) may vary from the implementation when other scaffolds are used.

Start by reading the discussions on #4271 and inspecting the current Android app-menu, toolbar, and default Scaffold behavior. Compare that implementation with the iOS UIMenu approach. Done means the App menu is available from the top-left navigation-bar icon on both platforms, with Android's About item migrated to the App group and iOS menus implemented.

Written by the indexing model from the issue text.