Move Demo Mode prefs to ash/constants/ash_pref_names.h.

This is in prepare for accessing Demo Mode prefs in
chromeos/ash/components.

Bug: b/298467438
Change-Id: I81b7c2f3dc3b1b33758ae888995100719798bb5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4868653
Commit-Queue: Li Lin <llin@chromium.org>
Reviewed-by: Jackson Tadie <jacksontadie@google.com>
Reviewed-by: Yilkal Abe <yilkal@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1198625}