add system group to netmgrd process

To allow netmgrd to acquire wakelocks correctly
we need to ensure that it belongs to system group

Bug: 24546055
Change-Id: I6231c7d1eb57a12da9be4f081e085e6f1d43b797
diff --git a/init.bullhead.rc b/init.bullhead.rc
index 7936caf..d6974de 100644
--- a/init.bullhead.rc
+++ b/init.bullhead.rc
@@ -281,7 +281,7 @@
 
 service netmgrd /system/bin/netmgrd
     class main
-    group radio
+    group radio system
 
 service per_mgr /system/bin/pm-service
     class core