keyboard: Reset height before requesting preferred height

keyboard: Reset height before requesting preferred height

If we don't set the height property to -1 before requesting the
preferred height, get_preferred_height() will just return us the fixed
height that has been set before. We don't want this behavior here, so
set the height to -1 before calling get_preferred_height().

This fixes a resizing issue where the keyboard is sized incorrectly
after switching the monitor into portrait mode and back.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2306>