PHILADELPHIA — The former Philadelphia Police officer who shot a 12-year-old boy in the back in March has been charged with murder after prosecutors said he fired the fatal shot from near-point blank range while the boy was on the ground and unarmed. Edsaul Mendoza, 26, surrendered Sunday night and faces counts including first- and third-degree murder, voluntary manslaughter, and possessing an instrument of crime in the death of Thomas “TJ” Siderio, court records show. Mendoza was jailed and denied bail. Philadelphia District Attorney Larry Krasner said at a news conference that the shooting w…