Complete implementation of menus and toolbars on mobile

[freakboy3742]

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.

[johnzhou721]

See #4048 for a first attempt at an implementation; howver, we'd need to get a coherent set of design decisions sorted out first. If someone else is up for resolving those issues, #4048 may inform you of the native APIs available.