[ios] Add new fullscreen provider API

The fullscreen implementation depends on the existence of a service
implemented in the internal repository. Add a function exposing if
the service is available.

This is part of a multi CLs change:
 => https://crrev.com/c/3819785
 -  https://crrev.com/i/4924057
 -  https://crrev.com/c/3823162

Bug: 1345810, 1201182
Change-Id: I847425ca82f6e05a6bba8782614df2e73c514643
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3819785
Reviewed-by: Aliona Dangla <alionadangla@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1033497}