libnm-gtk: use Bluetooth name for disambiguation

Currently if we have two paired bluetooth devices,
nma_utils_disambiguate_device_names() will disambiguate them by their
hardware address.  This is not very helpful, so detect this case and use
the Bluetooth device name instead.

This function is used by System Settings when showing the list of
network devices.

https://bugzilla.gnome.org/show_bug.cgi?id=740553