Introduce asec_public_file type.

This new type will allow us to write finer-grained
policy concerning asec containers. Some files of
these containers need to be world readable.

Signed-off-by: rpcraig <rpcraig@tycho.ncsc.mil>

(cherry picked from commit 48b18832)

Change-Id: Ic75095397a11ad715c16a75a7374e9b0d131f3f7