Introduce system_file_type

system_file_type is a new attribute used to identify files which exist
on the /system partition. It's useful for allow rules in init, which are
based off of a blacklist of writable files. Additionally, it's useful
for constructing neverallow rules to prevent regressions.

Additionally, add commented out tests which enforce that all files on
the /system partition have the system_file_type attribute. These tests
will be uncommented in a future change after all the device-specific
policies are cleaned up.

Test: Device boots and no obvious problems.
Change-Id: Id9bae6625f042594c8eba74ca712abb09702c1e5
diff --git a/public/netd.te b/public/netd.te
index a4a65a9..241380b 100644
--- a/public/netd.te
+++ b/public/netd.te
@@ -1,6 +1,6 @@
 # network manager
 type netd, domain, mlstrustedsubject;
-type netd_exec, exec_type, file_type;
+type netd_exec, system_file_type, exec_type, file_type;
 
 net_domain(netd)
 # in addition to ioctls whitelisted for all domains, grant netd priv_sock_ioctls.