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/traced_probes.te b/private/traced_probes.te
index 83dbe45..e173293 100644
--- a/private/traced_probes.te
+++ b/private/traced_probes.te
@@ -1,5 +1,5 @@
 # Perfetto tracing probes, has tracefs access.
-type traced_probes_exec, exec_type, file_type;
+type traced_probes_exec, system_file_type, exec_type, file_type;
 
 # Allow init to exec the daemon.
 init_daemon_domain(traced_probes)