storaged: assign to system-background cpuset

Bug: 34388593
Change-Id: I11d904cade1d67e54b5e651b6fc1ad34d365690e
diff --git a/storaged/storaged.rc b/storaged/storaged.rc
index 5fdbb8a..a8fa5a7 100644
--- a/storaged/storaged.rc
+++ b/storaged/storaged.rc
@@ -2,3 +2,4 @@
     class main
     file /d/mmc0/mmc0:0001/ext_csd r
     group root readproc
+    writepid /dev/cpuset/system-background/tasks