CredentialManagers emit a user-authenticated signal with a token
upon a successful authentication. This marks the service as preempting
and will be used for the next login/unlock request. However, the
preempting service is not cleaned after a successful verification.
As a consequence, it will be picked on a second unlock request.
This doesn't happen after a succesful login request because there
is a transition from the gnome-shell process owened by GDM to the
one owned by the user.
Clean the preempting service on a successful verification, like we
do when the PAM conversation stops.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3291>
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