ControlsManagerLayout: Use the workarea size to compute the available height

ControlsManagerLayout: Use the workarea size to compute the available height

To compute the available height for the layout we're currently using the
panel position, while this works for the current and default setup, the
shell may be configured to use a different workarea, so we should rely on
it to compute the available space, instead of a specific widget.

So get the current monitor index for the current view and use its coordinates
instead.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1892>