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/atrace.te b/private/atrace.te
index 1b86d3e..ac9bedb 100644
--- a/private/atrace.te
+++ b/private/atrace.te
@@ -2,7 +2,7 @@
 # It is spawned either by traced_probes or by init for the boottrace service.
 
 type atrace, domain, coredomain;
-type atrace_exec, exec_type, file_type;
+type atrace_exec, exec_type, file_type, system_file_type;
 
 # boottrace services uses /data/misc/boottrace/categories
 allow atrace boottrace_data_file:dir search;