automountManager: Cancel operations on disable

automountManager: Cancel operations on disable

We stop monitoring volumes when disabling automounts, but active
operations remain in place. As a result, it is possible for mount
password dialogs to show on the lock screen.

This is at least unexpected, so use a cancellable to make sure
ongoing operations are aborted when the automount manager is
disabled.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4393>
(cherry picked from commit fea948928fe5885c1f0e8c5b928d753dfb1c57a8)