Put statsd on system-background cpu set.

Bug: 72176997
Test: manual
Change-Id: Ic160dc8ffea8fd349185ad8b583e6c02dc7defa6
diff --git a/cmds/statsd/statsd.rc b/cmds/statsd/statsd.rc
index c260ae1..920273b 100644
--- a/cmds/statsd/statsd.rc
+++ b/cmds/statsd/statsd.rc
@@ -16,6 +16,7 @@
     class main
     user statsd
     group statsd log
+    writepid /dev/cpuset/system-background/tasks
 
 on post-fs-data
     # Create directory for statsd