overview: Fix delayed grab when entering via gesture

overview: Fix delayed grab when entering via gesture

When showing the overview in response to a gesture update, we
set the corresponding overview properties first before syncing
the grab.

However as we don't sync the grab while the overview is animating,
we currently end up only taking the grab once the animation is complete.

This is unintentional and messes with the expected keyboard focus,
so only mark the animation as in progress after syncing the grab.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/8967
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4128>