Default [Q] switch to art runtime

hi i have lg g pro lite wiith android 4.4.3, in developer options i don't have a runtime option.
So i have tried to change it manually, i modified /data/property/persist.sys.dalvik.vm.lib and to build.prop i add this:
After that i reboot my device, it take more than usually but dalvik cache is still i have a trouble....after reboot the /data/property/persist.sys.dalvik.vm.lib file contains again!?

Anyone can help me?