Nearly 15,000 mostly Asian-American protesters rallied in Brooklyn Saturday for former NYPD Officer Peter Liang, claiming that the rookie cop was a “pawn” of anti-police politics and was wrongly prosecuted for a tragic accident.The crowd filled Cadman Plaza Park, with many carrying signs with slogans like, “One tragedy, two victims” and, “Scapegoating won’t bring peace.” Many placards bore Martin Luther King Jr.’s photo and quote, “Injustice anywhere is