I don't believe this Joker told the police about the bomb in his apt. I think when he was arrested he made some comment like "Did my bomb take down the building?" which alerted the cops to look for the bomb. I don't think this Joker all of a sudden decided to "be nice and spare some cops" after his shooting spree.
I heard earlier on the nightly news that he told LE there may be explosives in his apartment. Now whether he came out and said that, or it was an answer to one of their questions, they did not say. I do not think he told them there were booby traps set up. I think he still wanted them to go to the door and check it out, they were just smarter than that.