Move blkid policy to private

This leaves only the existence of blkid and blkid_untrusted domains as
public API. All other rules are implementation details of these
domains' policy and are thus now private.

Test: No change to policy according to sesearch, except for
      disappearance of all allow rules to do with blkid_current and
      blkid_untrusted_current (as expected).
Bug: 31364497
Change-Id: I0dda2feeb64608b204006eecd8a7c9b9c7bb2b81