NEW YORK — Tossing the gavel aside, feisty judge Shira Scheindlin is speaking out, declaring victory over critics in the battle over police stop-and-frisk tactics and telling why she thinks the government spoiled its chance at conviction in four trials of John “Junior” Gotti.Scheindlin ended a 25-year career on the bench Friday to join a law firm, hoping to find public interest work to improve things like prison reform, voting rights and medical care.She said she isn’t leaving the lifetime …