add system-background cpuset

add a new cpuset for system services that should not run on
larger cores

bug 24144797

Change-Id: I4f3cd34711b7ffcd2ef45ef2a3cd4642d0ef34c5
diff --git a/services/surfaceflinger/surfaceflinger.rc b/services/surfaceflinger/surfaceflinger.rc
index 718b6b65..59a43e2 100644
--- a/services/surfaceflinger/surfaceflinger.rc
+++ b/services/surfaceflinger/surfaceflinger.rc
@@ -3,3 +3,4 @@
     user system
     group graphics drmrpc
     onrestart restart zygote
+    writepid /dev/cpuset/system-background/tasks