update_engine: put update_engine into background sched group

update_engine runs as 120 may impact foreground tasks, put it into bg
sched group to have soft limit on its cpu usage.

Bug: 194838704
Test: build
Signed-off-by: Wei Wang <wvw@google.com>
Change-Id: Ifa66f0119e86d79fa48d15a82b192f33b6ab2740

update_engine.rc

diff --git a/update_engine.rc b/update_engine.rc
index b9f80fc..aa39211 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 /dev/blkio/background/tasks
+    task_profiles ServiceCapacityLow LowIoPriority HighEnergySaving
     disabled
 
 on property:ro.boot.slot_suffix=*