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

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

Change-Id: Ifba71d073dc7b577911386cff316e8a19662cd0d
selinux: When adding userdata labels, do cache too

Change-Id: I0c1ff737629474135767be7cfe2bfbea0513fa9e