Matty’s back! Days after The Post exposed the military’s wrongful, 16-month-long separation of injured Army Spc. Brent Grommet and his war dog, Matty, the two were finally reunited on Friday. “I’m about the happiest I’ve ever been,” Grommet, 23, told The Post. Grommet and Matty, who specialized in detecting IEDs, were injured when one exploded near them in Afghanistan. Grommet came back with PTSD, traumatic brain injury, migraines, severe chronic back pain and breathing problems. Under Robby’s Law — signed by Bill Clinton in 2000 — military dog handlers have the first right to adopt their animals if injured together....