Move servicemanager into system-background cpuset.

Bug: 24949295
Change-Id: I9149c43edf79d9acbc5af4b3dc1eae59c695a86a
diff --git a/cmds/servicemanager/servicemanager.rc b/cmds/servicemanager/servicemanager.rc
index 1ba339d..aee7bd8 100644
--- a/cmds/servicemanager/servicemanager.rc
+++ b/cmds/servicemanager/servicemanager.rc
@@ -11,4 +11,5 @@
     onrestart restart inputflinger
     onrestart restart drm
     onrestart restart cameraserver
+    writepid /dev/cpuset/system-background/tasks