By MICHAEL R. SISAK UNITED NATIONS (AP) — Below United Nations headquarters, a state-of-the-art security post dubbed the “Brain Center” hums with activity on the eve of next week’s high-level meeting of the U.N. General Assembly. The annual diplomatic pilgrimage is bringing more than 140 world leaders to New York City, including the heads of Israel, Palestine and Ukraine. Keeping them safe is the U.S.