bluetooth: Add missing 'BluetoothHardwareAirplaneMode' to D-Bus interface

This D-Bus property was never been added here, which caused inconsistencies
under some scenarios (e.g. coming back from suspend) if some devices were
previously paired, since _sync() would then make the bluetooth menu visible
unconditially, because of the proxied property evaluating to 'false'.

Adding this to the D-Bus interface makes sure that it's no longer undefined
and returns the right value, fixing the bug.

https://bugzilla.gnome.org/show_bug.cgi?id=789110