Add support for background stune group.

bug 29512132

Change-Id: If8144bfee0fb30cf11f2bb26494ca5e83c11d4d7
diff --git a/libcutils/sched_policy.c b/libcutils/sched_policy.c
index c097a12..05a2048 100644
--- a/libcutils/sched_policy.c
+++ b/libcutils/sched_policy.c
@@ -145,7 +145,7 @@
         ta_schedboost_fd = open(filename, O_WRONLY | O_CLOEXEC);
         filename = "/dev/stune/foreground/tasks";
         fg_schedboost_fd = open(filename, O_WRONLY | O_CLOEXEC);
-        filename = "/dev/stune/tasks";
+        filename = "/dev/stune/background/tasks";
         bg_schedboost_fd = open(filename, O_WRONLY | O_CLOEXEC);
 #endif
     }