Move vold (and sdcard childs) to foreground cpuset.

Bug: 24949295
Change-Id: I7118a374db90ede517a92f72763d624b9a234ea7
diff --git a/vold.rc b/vold.rc
index 7719e0b..53ea7fc 100644
--- a/vold.rc
+++ b/vold.rc
@@ -5,3 +5,4 @@
     socket vold stream 0660 root mount
     socket cryptd stream 0660 root mount
     ioprio be 2
+    writepid /dev/cpuset/foreground/tasks