Protect /data/property.

/data/property is only accessible by root and is used by the init
property service for storing persistent property values.  Create
a separate type for it and only allow init to write to the directory
and files within it.  Ensure that we do not allow access to other domains
in future changes or device-specific policy via a neverallow rule.

Change-Id: Iff556b9606c5651c0f1bba902e30b59bdd6f063a
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>