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/private/mdnsd.te b/private/mdnsd.te
index 943f979..98e95da 100644
--- a/private/mdnsd.te
+++ b/private/mdnsd.te
@@ -3,7 +3,7 @@
 typeattribute mdnsd coredomain;
 typeattribute mdnsd mlstrustedsubject;
 
-type mdnsd_exec, exec_type, file_type;
+type mdnsd_exec, system_file_type, exec_type, file_type;
 init_daemon_domain(mdnsd)
 
 net_domain(mdnsd)