Modularize side panel

This CL is a refactor with no intended behavior change.

This CL moves side panel C++ code into a standalone module. This CL
removes //chrome/browser/companion from android, which was only included
by accident.

Change-Id: I37dad5bc447e995d975511aced99f8f05a30eec1
Bug: 345499793
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5607343


Reviewed-by: Caroline Rising <corising@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313033}