[autopip] Create settings UI helper class

This CL introduces a helper class to hold all the content setting
and embargo logic for auto-pip.  Based on the content setting, it
will optionally create the "allow / block" UI to let the user
change the content setting from the pip window.

It also introduces the plumbing to keep this logic shared between
the video and document pip implementations.

Right now, it has only placeholder logic to display the auto-pip
allow / block UI at all times when the off-by-default Media Session
pip action flag is enabled, for document pip only.  Video pip does
not display the UI, to keep the CL size more manageable.

Change-Id: If94f43fec14db22e84983d1dd19b12be5ca72fd2
Bug: 1464066
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4688277


Commit-Queue: Frank Liberato <liberato@chromium.org>
Reviewed-by: Fr <beaufort.francois@gmail.com>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1184846}