The United States military imposed a curfew on Friday on all of its nearly 50,000 uniformed personnel stationed in Japan, as it tried to respond to public outrage over the suspected rape of a woman in Okinawa by two American sailors. The case struck a nerve in Okinawa, with its uncomfortable echoes of the 1995 gang rape of a schoolgirl by three American servicemen. That crime prompted huge demonstrations that for a time seemed to threaten the entire American military presence on the island.