[CrOS MultiDevice] Move appropriate CryptAuth files to SecureChannel.

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.

Bug: 912743
Change-Id: Ica14efe50b0c0c3ca85802fe969afff7e7d26f9a
Reviewed-on: https://chromium-review.googlesource.com/c/1372052
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616383}
121 files changed