init: Start update_engine by default as root.

This patch starts update_engine daemon by default and running as root.

Bug: 23186405
Test: booted dragonboard, update_engine starts automatically. Successfully applied a payload v2 (no postinstall enabled).

Change-Id: I2c4024292f9eb45a70fc44f7f3e612728e910df5
diff --git a/update_engine.rc b/update_engine.rc
index fe5b224..71f918f 100644
--- a/update_engine.rc
+++ b/update_engine.rc
@@ -1,8 +1,4 @@
-on boot
-    mkdir /data/misc/update_engine 0700 system system
-
 service update_engine /system/bin/update_engine --logtostderr --foreground
     class late_start
-    user system
-    group system dbus inet
-    disabled
+    user root
+    group root dbus inet