Install offline Demo Mode policy from offline resources

During offline Demo Mode setup, install the policy files from the
preinstalled offline resources, rather than rootfs.

In CrOS, the files will be removed from rootfs and added to the
offline-demo-mode-resources component.

Bug: 886973
Change-Id: Ia947c9f1d2c2df2da997539c581338efeb82a814
Reviewed-on: https://chromium-review.googlesource.com/c/1407601
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622552}
7 files changed