Grant "disk_reserved" GID to critical services.

This GID extends the ability to use reserved disk space, giving the
system a chance to be usable enough for the user to free up disk
space used by apps.

Test: builds, boots
Bug: 62024591
Change-Id: I8bc47911a71e1f399616caae83678e2914781c7e
diff --git a/vold.rc b/vold.rc
index c27aeda..7d14453 100644
--- a/vold.rc
+++ b/vold.rc
@@ -5,3 +5,4 @@
     ioprio be 2
     writepid /dev/cpuset/foreground/tasks
     shutdown critical
+    group reserved_disk