network: Sync airplane mode status on startup

https://bugzilla.gnome.org/show_bug.cgi?id=706042
network: Use HasAirplaneMode to determine whether to show the switch

Not all systems have rfkill toggles we can set; if we don't have any,
we shouldn't show any switch at all.

https://bugzilla.gnome.org/show_bug.cgi?id=706042