bluetooth: Use a separate signal for changes to the default adapter

In gnome-bluetooth API version 1.0 (GNOME 41 and older), the Bluetooth
adapters are part of the same GtkTreeModel as the devices attached to
them, but in gnome-bluetooth API version 3.0 (GNOME 42) they're tracked
separately.

Signed-off-by: Simon McVittie <smcv@debian.org>