Allow postinstall_file to be an entrypoint.

postinstall_file was an exec_type so it could be an entrypoint for the
domain_auto_trans from update_engine domain to postinstall domain. This
patch removes the exec_type from postinstall_file and exempts it from
the neverallow rule to become an entrypoint.

Bug: 28008031
TEST=postinstall_example still runs as the "postinstall" domain on edison-eng.

Change-Id: Icbf5b262c6f971ce054f1b4896c611b32a6d66b5