The attribute is used to capture system properties added from outside of
AOSP (e.g. by OEM), but are not device-specific and thus are used only
inside the system partition.

Access to the the system properties from outside of the system partition
is prevented by the neverallow rule.

Bug: 80382020
Bug: 78598545
Test: m -j selinux_policy
Change-Id: I22c083dc195dab84c9c21a79fbe3ad823a3bbb46