[MirrorSync] Add dialog to manage MirrorSync settings

MirrorSync will be available via a few entry points so follow the same
method as the SMB shares dialog to ensure the various entry surfaces can
load the dialog. This adds a basic skeleton dialog with a Mojo pipe that
will be used to access DriveFS interface and receive messages from the
delegate.

Bug: b:237066325
Test: chromeos-on-linux and ensure chrome://manage-mirrorsync loads
Test: Disable flag and ensure it doesn't load
Change-Id: If96a4a775f7db5526acda572b54df389056b16c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3722143
Reviewed-by: Christopher Lam <calamity@chromium.org>
Commit-Queue: Ben Reich <benreich@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1019455}