[Refactor] Modulize SingleTabSwitcher code.

In this CL, we move SingleTab* related code to
chrome/browser/single_tab. There isn't any behaviour change.

Change-Id: I3d71ca59cc8e5e1cff29c0bc19b4b9a900719c0a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5404167


Reviewed-by: Theresa Sullivan <twellington@chromium.org>
Commit-Queue: Xi Han <hanxi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1279882}