selinux changes for DNS metrics.

1. Allow the system server to create the dns_listener service.
2. Allow netd to use said service.

Change-Id: Ic6394d7b2bdebf1c4d6cf70a79754a4996e943e2
diff --git a/service_contexts b/service_contexts
index 288ff90..11c0736 100644
--- a/service_contexts
+++ b/service_contexts
@@ -34,6 +34,7 @@
 diskstats                                 u:object_r:diskstats_service:s0
 display.qservice                          u:object_r:surfaceflinger_service:s0
 display                                   u:object_r:display_service:s0
+dns_listener                              u:object_r:dns_listener_service:s0
 DockObserver                              u:object_r:DockObserver_service:s0
 dreams                                    u:object_r:dreams_service:s0
 drm.drmManager                            u:object_r:drmserver_service:s0