Revert "[QuickStart] Move handshake code to oobe_quick_start/connectivity"

This reverts commit ff7de2e9.

Reason for revert: Causing unit tests failing.
https://ci.chromium.org/ui/p/chromium/builders/ci/linux-chromeos-dbg/b8780340991630105425/overview


Original change's description:
> [QuickStart] Move handshake code to oobe_quick_start/connectivity
>
> Since we're now planning to implement handshake parsing on the browser
> process, the existing handshake code no longer needs to be accessible
> outside the connectivity directory.
>
> Also make BuildHandshakeMessage() responsible for generating the nonce.
> This removes a lot of awkward code used to inject a nonce in tests.
>
> Bug: b/279612583
> Test: Tests added in crrev.com/c/4520128
> Change-Id: I3ea03241f29e6a13691ab08ccdd43a0141524825
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4520834


> Reviewed-by: Brittany Hartmire <bhartmire@google.com>
> Reviewed-by: Joe Antonetti <joeantonetti@google.com>
> Commit-Queue: Curt Clemens <cclem@google.com>
> Cr-Commit-Position: refs/heads/main@{#1147325}

Bug: b/279612583, 1448031
Change-Id: Iec878b645104eac57e2fb6a8f631828ebc79f795
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4557688
Owners-Override: Owen Min <zmin@chromium.org>
Auto-Submit: Owen Min <zmin@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1148008}