Move gatekeeperd to system-background cpuset.

Bug: 24949295
Change-Id: I3c68bf1c604f0e8eb8155a4615802847a9a8604d
diff --git a/gatekeeperd/gatekeeperd.rc b/gatekeeperd/gatekeeperd.rc
index 3f1b92d..8b126d5 100644
--- a/gatekeeperd/gatekeeperd.rc
+++ b/gatekeeperd/gatekeeperd.rc
@@ -1,3 +1,4 @@
 service gatekeeperd /system/bin/gatekeeperd /data/misc/gatekeeper
     class late_start
     user system
+    writepid /dev/cpuset/system-background/tasks