sepolicy: allow system_server to read /proc/locks

Access to /proc/locks is necessary to activity manager to determine
wheter a process holds a lock or not prior freezing it.

Test: verified access of /proc/locks while testing other CLs in the same
topic.
Bug: 176928302

Change-Id: I14a65da126ff26c6528edae137d3ee85d3611509
diff --git a/private/system_server.te b/private/system_server.te
index 95d7cc7..12fb3fa 100644
--- a/private/system_server.te
+++ b/private/system_server.te
@@ -937,6 +937,7 @@
 allow system_server {
   proc_cmdline
   proc_loadavg
+  proc_locks
   proc_meminfo
   proc_pagetypeinfo
   proc_pipe_conf