Add support for background stune group.

bug 29512132

Change-Id: If8144bfee0fb30cf11f2bb26494ca5e83c11d4d7
diff --git a/rootdir/init.rc b/rootdir/init.rc
index 32970e4..97bfe6b 100644
--- a/rootdir/init.rc
+++ b/rootdir/init.rc
@@ -50,15 +50,19 @@
     mkdir /dev/stune
     mount cgroup none /dev/stune schedtune
     mkdir /dev/stune/foreground
+    mkdir /dev/stune/background
     mkdir /dev/stune/top-app
     chown system system /dev/stune
     chown system system /dev/stune/foreground
+    chown system system /dev/stune/background
     chown system system /dev/stune/top-app
     chown system system /dev/stune/tasks
     chown system system /dev/stune/foreground/tasks
+    chown system system /dev/stune/background/tasks
     chown system system /dev/stune/top-app/tasks
     chmod 0664 /dev/stune/tasks
     chmod 0664 /dev/stune/foreground/tasks
+    chmod 0664 /dev/stune/background/tasks
     chmod 0664 /dev/stune/top-app/tasks
 
     # Mount staging areas for devices managed by vold