download_gki: Add support for signed boot-user.img

xTS testing requires a signed user boot.img. Now that these are more
readily available, default to downloading the user boot.img from the
signed artifacts. Support for unsigned boot-user.img remains via a
switch.

Bug: 194513402
Signed-off-by: J. Avila <elavila@google.com>
Change-Id: I9b8cd2b746fb6aac7323379d865d1c3507ae1de6