overview: Start animation after setting coverPane initial state

Since commit 7bd98f3f the onComplete callback
is called right away when animations are disabled. As side effect, now
this._coverPane gets shown/hidden in the wrong order. Start the animation after
setting this._coverPane initial state, so that the animation callback can set the
this._coverPane final state without it being overwritten later.

Move the animation also after emitting the showing and hiding signals. So that the
order showing -> shown and hiding -> hidden is still preserved when animations are
disabled.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/6096
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2532>
(cherry picked from commit cf47b4a3)