rfkill: Account for having zero rfkill switches for airplane mode

This is the case in desktops and VMs.

https://bugzilla.gnome.org/show_bug.cgi?id=706041
rfkill: Add a new property to determine whether we can toggle airplane mode

On some systems like desktops or VMs, we have no rfkill switches to toggle,
so we shouldn't show any switches in the control center.

https://bugzilla.gnome.org/show_bug.cgi?id=706041