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This CL adds a way to suppress (and un-suppress) picture-in-picture
windows. If it is set, picture-in-picture windows can't be crated. If
any picture-in-picture windows exist already, they are closed.

This CL suppresses only ARC PIP window. Chrome PIP window will be done
separately.

Summary of change of this CL:
- Makes ArcPipBridge observe spoken-feedback pref change
- Sends a newly-added mojo message when the spoken feedback en/disables

Related CLs:
- crrev.com/c/1285253 (chromium/src)
- ag/5287718 (vendor/google_arc)
- ag/5287717 (frameworks/base)

Bug: b/112060145
Test: manual (see pip window is supppressed correctly)
Change-Id: I554d74672e4e5136ae21fc0d2440a4150e5b5608
Reviewed-on: https://chromium-review.googlesource.com/c/1285253Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Reviewed-by: Eliot Courtney <edcourtney@chromium.org>
Commit-Queue: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607489}