Migrate //ash/services/multidevice_setup to namespace ash [1/3]

This CL changes namespace of mojom and proto to ash from chromeos.

This CL is part of the Chrome OS source code directory migration:
https://docs.google.com/document/d/1g-98HpzA8XcoGBWUv1gQNr4rbnD5yfvbtYZyPDDbkaE

Bug: 1164001
Change-Id: Ifc5d23dd9c193947e990327e42b83615801c133a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3484035
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Owners-Override: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Yeunjoo Choi <ychoi@igalia.com>
Cr-Commit-Position: refs/heads/main@{#977481}