run update_engine in the background

Put update_engine in system-background cgroup

Bug: 27246627
Change-Id: Ia7cdb2eb226f6076d20e447eea2a04e4c83ee0f3
diff --git a/update_engine.rc b/update_engine.rc
index 499a8f6..62bbcc7 100644
--- a/update_engine.rc
+++ b/update_engine.rc
@@ -2,3 +2,4 @@
     class late_start
     user root
     group root system wakelock dbus inet
+    writepid /dev/cpuset/system-background/tasks