It's very easy to wait for property change both in Java and C++,
but it's not the case in shell script. With this patch, developers
can now easily to wait for property change, just like what we have
in `.rc` files, and to wait for boot complete.
ro properties' triggers should only be triggered once, otherwise it
may undefined behaviour.
This patch avoids triggering ro properties' actions again when using
resetprop to modify them.
Co-authored-by: 5ec1cff <ewtqyqyewtqyqy@gmail.com>