Put update_engine to blkio background group
am: 13bdba8cdc

Change-Id: I6664f84fe4cddfc3ff203506108f56b3f4a39474
diff --git a/update_engine.rc b/update_engine.rc
index 90ca4c6..b9f80fc 100644
--- a/update_engine.rc
+++ b/update_engine.rc
@@ -2,7 +2,7 @@
     class late_start
     user root
     group root system wakelock inet cache media_rw
-    writepid /dev/cpuset/system-background/tasks
+    writepid /dev/cpuset/system-background/tasks /dev/blkio/background/tasks
     disabled
 
 on property:ro.boot.slot_suffix=*