scanning: change DLC path

The scanning notification controller and DLC predownload list are both
using the sane-backends-extras-dlc path, but the new scan DLC is located
at sane-backends-pfu.

Bug: b:328790441
Change-Id: I911f7b5084bd0ae12af3dc707250a8dad64503a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5354990


Reviewed-by: Anqing Zhao <anqing@chromium.org>
Reviewed-by: Benjamin Gordon <bmgordon@chromium.org>
Commit-Queue: Paul Moy <pmoy@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1270666}