[Enhancement] Make ActivityIndicator fullscreen for appshell

[leandrofagundes]

Summary

When using activity indicator on a ShellApp project, the NavBar stills unblocked and the user can navigate back or navigate through menu even when app is busy.

API Changes

In order to allow the activity indicator to cover full screen, even the nav bar, a template can be add to the appshell.xaml configuration for when the application is busy.

Intended Use Case

Every time we need a loading on the screen, we can just call Shell.IsBusy or something like that and the template will be activated.

[PureWeen]

@leandrofagundes what do you think about this approach?

#8551

[leandrofagundes]

Thanks @PureWeen, it really works. I just think it could be a template on appshell.xaml.