Demo Mode: Preload offline resources during setup

Demo Mode setup should attempt to load offline resources at the network
screen, so that we only offer Offline Demo Mode as an option when the
offline resources dm-verity image actually exists.

This CL updates the preinstalled offline resources image location
(offline-demo-mode-resources) and changes the network screen to wait for
the resources to be mounted before showing the offline Demo Mode option.

Bug: 904641
Change-Id: I7a38fbb5b4a2c1c7ad28f9ef0945a2fb7cd09259
Reviewed-on: https://chromium-review.googlesource.com/c/1403181
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Wenzhao (Colin) Zang <wzang@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622293}
20 files changed