Allow priv_apps to stat files on the system partition

Allows safetynet to scan the system partition which is made up of
files labeled system_file (already allowed) and/or files with the
exec_type attribute.

Bug: 25821333
Change-Id: I9c1c9c11bc568138aa115ba83238ce7475fbc5e4

priv_app.te

@@ -60,6 +60,10 @@ userdebug_or_eng(`
   allow priv_app perfprofd_data_file:dir r_dir_perms;
 ')
 
+# Allow GMS core to stat files and executables on
+# the system partition
+allow priv_app exec_type:file getattr;
+
 ###
 ### neverallow rules
 ###