bin/backlight_helper: don't release global udev context

1. can't unref udev, because the udev_deivce created by
   udev_device_new_from_syspath is dependent it.
2. use sync.Mutex to ensure safety

tower: https://tower.im/projects/8f19f0bf0e754f0b82ef2c24bc230973/todos/fd6b074e64c94faebe6f20cae2ceb3b5/

Change-Id: I69fbbf9fef624ef84ae423730eee724dc1c9793f