Conditionally set dbus group in init rc.

Change-Id: I9958b56502ee5eecdb3ca279779c6a66774b88f5
diff --git a/update_engine.rc b/update_engine.rc
index 69fc7af..e62d83c 100644
--- a/update_engine.rc
+++ b/update_engine.rc
@@ -1,4 +1,4 @@
 service update_engine /system/bin/update_engine --logtostderr --foreground
     class late_start
     user root
-    group root system dbus inet
+    group root system inet