Do not permit appdomain to create/write to download_file.

The comment says that apps can read downloaded files, but the
file_type_auto_trans() macro expands to permit create/write access.
Also we don't need a type transition when staying in the same type
as the parent directory so we only truly need allow rules here.
Hence, we remove file_type_auto_trans() altogether, and add an allow
rule for search access to the directory.  If create/write access is
truly required, then we can just change the allow rules to use
rw_dir_perms and create_file_perms.

Change-Id: Icd71c9678419442cfd8088317317efd4332f9b4a
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>