[CrOS MultiDevice] Move SecureMessageDelegate et. al to MultiDevice.

This is part of the ongoing effort to clean up the directory structure
of multi-device code. The files moved are only relevant to the
DeviceSync service, so they belong within the service implementation.

See go/multidevice-directory-structure-cleanup for details.

TBR=blundell@chromium.org

Bug: 912743
Change-Id: I14fe432d62e86219227554b4e454a975f2859e84
Reviewed-on: https://chromium-review.googlesource.com/c/1375370
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616408}
45 files changed