overviewControls: Set expandFraction for ws thumbnails to 0 when hidden

overviewControls: Set expandFraction for ws thumbnails to 0 when hidden

We multiply the spacing around the workspace thumbnails with the
expandFraction so that it grows with the animation to show/hide them. We
shouldn't factor in the extra spacing to accomodate for the thumbnails in
case they're hidden though, so make sure expandFraction is set to 0 after
the thumbnails got hidden.

This fixes the spacing when a user never uses workspaces (and the workspace
thumbnails are never shown), and also the first transition from
hidden->shown for the thumbnails, which currently starts at 100% scale (ie.
pops into existence).

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3278>