update_engine.rc: Add update_engine to system group.

update_engine needs to create the uma-events file in the directory
/data/misc/metrics which is owned by system:system. Adding it back to
the system group allows update_engine to report metrics.

Bug: 24989383
Test: make; provision; deployed an update and didn't trigger the reported SELinux errors anymore.

Change-Id: I65248a80a7019a21e8b26c4117c43308e6cd48a2
diff --git a/update_engine.rc b/update_engine.rc
index 71f918f..69fc7af 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 dbus inet
+    group root system dbus inet