[live-preview] Add PermissionPromptPreviewsWrapper

Delegate MediaCoordinator initialization to a wrapper to facilitate
metrics collection. Also, this would help handling custom behaviour
expected in the future.

Bug: b:332613262
Change-Id: I18b59a7b8cc4485f809c27cc4cb254f537a46822
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5422050


Reviewed-by: Bryant Chandler <bryantchandler@chromium.org>
Commit-Queue: Ahmed Moussa <ahmedmoussa@google.com>
Cr-Commit-Position: refs/heads/main@{#1282801}