I'm afraid by extending the flaps in this matter the entirely opposite would occur. Sure extending the flap beyond the 20 degrees or so would increase the drag but as flap increases lift increases continually. Sure the drag increases but drag is a minor contributor to stopping a C172 at such a low speed. The way to stop the aircraft fast is friction between the tyres and ground and braking. With an increase in lift, there is less weight on the main wheels, less friction and less braking effectiveness, and more stopping distance. In fact the technique used when landing on short strips in bush areas is, as soon as touching down from a short field technique landing is retract the flaps fully, full back pressure and maximum braking without skidding, and I can guarantee this will stop you the quickest.