NEW YORK - Maybe even more than other media bosses, David Janollari has to keep a moving target in his sights.Of course, wherever you look, the media landscape is in flux these days.But for Janollari, MTV's head of programming, the task is further complicated by the audience his network is aiming for: Young viewers who expect to get what they want when they want it, and whose tastes are constantly changing.To keep this crowd happy ...