overview: Skip SHOWN to SHOWN when changing from app grid to overview

overview: Skip SHOWN to SHOWN when changing from app grid to overview

Switching between the app grid and the window picker in the overview via
gestures results in _gestureEnd() getting called with endProgress !== 0
in both cases, which leads to it calling _showDone(). This then
unconditionally changes the state to SHOWN, which in this situation is
already the current state. Since the previous commit this results in a
warning, so check if we are already in the SHOWN state.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2514>