Get rid of separate download_file type.

This appears to have been created to allow untrusted_app to
access DownloadProvider cache files without needing to allow
open access to platform_app_data_file.  Now that platform_app_data_file
is gone, there is no benefit to having this type.

Retain a typealias for download_file to app_data_file until
restorecon /data/data support is in place to provide compatibility.

This change depends on:
https://android-review.googlesource.com/#/c/87801/

Change-Id: Iab3c99d7d5448bdaa5c1e03a98fb6163804e1ec4
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>