[FeedPreview] Handle when users change the "show link preview" setting

Set the status of the switcher according to feature flag and user's
pref. Handle user's action of toggling the switcher.

Bug: 1245823
Change-Id: Ia8d2c6129fdadbe806b79378aed77deac35ee7f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3140655


Commit-Queue: Tina Wang <tinazwang@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/main@{#918709}