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/genfs_contexts b/private/genfs_contexts
index 4c6edd6..900b25b 100644
--- a/private/genfs_contexts
+++ b/private/genfs_contexts
@@ -14,6 +14,7 @@
 genfscon proc /keys u:object_r:proc_keys:s0
 genfscon proc /kmsg u:object_r:proc_kmsg:s0
 genfscon proc /loadavg u:object_r:proc_loadavg:s0
+genfscon proc /locks u:object_r:proc_locks:s0
 genfscon proc /lowmemorykiller u:object_r:proc_lowmemorykiller:s0
 genfscon proc /meminfo u:object_r:proc_meminfo:s0
 genfscon proc /misc u:object_r:proc_misc:s0