selinux: Use a more reliable sentinel to flag /data labelling

/data/anr isn't present on all devices, /data/system is

Change-Id: Ifba71d073dc7b577911386cff316e8a19662cd0d
diff --git a/prebuilt/common/etc/init.d/50selinuxrelabel b/prebuilt/common/etc/init.d/50selinuxrelabel
index 4096fdc..be30775 100644
--- a/prebuilt/common/etc/init.d/50selinuxrelabel
+++ b/prebuilt/common/etc/init.d/50selinuxrelabel
@@ -12,7 +12,7 @@
 LABELSYS=0
 
 # Test /data
-ls -Zd /data/anr | grep -q unlabeled
+ls -Zd /data/system | grep -q unlabeled
 if [ $? -eq 0 ]; then
   $L "userdata is unlabeled, fixing..."
   LABELDATA=1