bluetooth: Fix showing menu when devices were set up

Since commit 26c2cb9f, nDevices is always the actual number of paired/trusted devices. So when bluetooth is turned off, it is now 0 rather than forced to 1 if devices were set up previously.

Fix this by checking the property that tracks set up devices instead.

(Note that this is only a fix as far as gnome-shell is concerned. Since gnome-bluetooth@c437c729 devices are removed before the adapter, so had-bluetooth-devices-set-up is always reset to false before we get to check it. This will need a fix on the gnome-bluetooth side as far as I can see ...)