[CrOS MultiDevice] Remove obsolete dependencies on MultiDevice services.

Now that code from Instant Tethering and Smart Lock has been fully
migrated to use the DeviceSync, SecureChannel, and MultiDeviceSetup
services, these features no longer need to access the implementations of
those services and should isntead use the client libraries and/or Mojo
APIs instead.

This CL removes the dependencies. Additionally, to prevent new code from
adding new dependencies, this CL updates the BUILD files for the
relevant services to deny visibility into the service implementations to
all code except for //chrome/browser (which initializes the services)
and test code (which mocks out the services).

Bug: 912743
Change-Id: Id8b58de73988b2142099adcd9fd8ef6f6584a11e
Reviewed-on: https://chromium-review.googlesource.com/c/1384931
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617963}
20 files changed