A JUDGE in Long Beach, Calif., declared a mistrial in the trial of two white former policemen charged in the arrest of a black activist. They were accused of pushing the activist through a window in an incident captured on videotape in a sting aimed at exposing police brutality and racism . . . A MEMBER of a prominent Protestant organization was killed Monday by an Irish Republican Army car bomb in Northern Ireland's town of Armagh . . . NASA HAS SET May 22 for the launch of space shuttle Columbia with seven astronauts, 30 rats and 2,400 tiny jellyfish. The nine-day mission, in which the astronauts will conduct medical tests on one another to gauge adaptation to space, will be the third shuttle flight in less than two months.