Don't leak hwservicemanager binders

Use defaultServiceManager() instead of IServiceManager::getService()

bug:62073349
test:
  repeat {launch Play Movies, play a movie, exit} while monitoring
    adb shell cat /sys/kernel/debug/binder/stats |
    grep -A10 '^proc <mediadrmserver PID>'
  --> Make sure number of nodes is stable

Change-Id: Ibea0437ed21a9d58f3eb7ffa9461fb71d9c2b4ab