LONDON — Gerry Adams, the divisive politician known around the world as the face of the Irish republican movement as it shifted from violence to peace, announced Saturday that he was stepping down as leader of Sinn Fein next year after heading the party for over 30 years. The 69-year-old veteran politician — who has been president of Northern Ireland’s second-largest party since 1983 — told the party’s annual conference in Dublin he will not run in the next Irish parliamentary elections. “Leadership means knowing when it is time for change and that time is now,” he said, adding the move was part of a process of leadership transition.