If you want to add/modify some system properties, it's an good idea to use SystemProperties.get & SystemProperties.set.
Note that the application should share the system uid, and have the signature of platform.
This example is creating a new system properties which would persist even if device reboot. Unless you erase User Data partition (such as performing factory reset)
After these three steps, you can find the property "persist.sys.demo" is
added in /data/property
About naming property, please follow the prefix that defined in system/core/init/property_service.c
There are some property file other than /data/property
Take a look if you're interested in it.