Strong Energy-Transfer-Induced Enhancement of Luminescence Efficiency of Eu2+- and Mn2+-Codoped Gamma-AlON for Near-UV-LED-Pumped Solid State Lighting (Strong Energy-Transfer-Induced Enhancement of Luminescence Efficiency of Eu2+- and Mn2+-Codoped Gamma-AlON for Near-UV-LED- Pumped Solid State Lighting)