Restrict the destination types read by the CUPS printing backends

Currently, only scanner devices are filtered out, whereas fax devices
are still read by the CUPS backends. As a consequence, many printers
that also serve as fax devices are shown as two separate destinations
in Print Preview. Stop accepting fax devices.

Similarly, stop accepting devices that are not discovered. Currently,
AirPrint destinations show up on macOS even if they're not added to the
system, and their capabilities cannot be read in that situation. This
way, users will be forced to add the AirPrint device on their systems
prior to seeing it on Print Preview, like they have to do with normal
local printers.

Bug: 1027834
Change-Id: Ib5b50465414a1214997f3aab3f770a1fc8467edb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2068868
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Sean Kau <skau@chromium.org>
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749865}