`ShouldShowAirplaneMode` only considers whether airplane mode
should be shown for the form factor, not whether there are
any actual kill switches available.
That's tracked in a separate property, `HasAirplaneMode`.
Take that into account for our `:show-airplane-mode` property,
so that it reflects when airplane mode should and *can* be shown.
Right now we only show airplane mode when it is enabled (and
therefore available), but this will change in the future.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2385>
