breakManager: Rework notification urgency setting and presentation

breakManager: Rework notification urgency setting and presentation

Move the setting of the notification’s urgency, and adding the
notification to the queue, to beside where the notification is created.

This avoids the potential for changing the urgency of an existing
notification if none of the branches above are taken, and
`this._notification` is non-empty.

Spotted by Florian Müllner in
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3687#note_2405781.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3687>