Media Effects Service - propagate model file to Video Effects Service

This change introduces MediaEffectsModelProvider that can be passed in
to Media Effects Service in order to allow it to subscribe to model
changes. The concrete implementation of MediaEffectsModelProvider in
turn uses OptimizationGuideKeyedService to subscribe to the model
updates. When the information about the model file for background
segmentation reaches MediaEffectsService, it will in turn propagate it
to Video Effects Service.

Bug: b:340042658
Change-Id: Iaf4951746d793b28ca59e61024b65fdd867c23a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5731497
Reviewed-by: Will Harris <wfh@chromium.org>
Commit-Queue: Piotr Bialecki <bialpio@chromium.org>
Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Reviewed-by: Bryant Chandler <bryantchandler@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1335238}