[iOS] Refactor the default browser exporter to be more generic

This CL moves and renames DefaultBrowserEventExporter to
feature_engagement to make it more generic and allow the migration of
events other than the default browser.

Bug: 394874024
Change-Id: I501384cad5f631e7bc10c40f3f171fe371d85c32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6263882
Commit-Queue: Cheick Cisse <cheickcisse@google.com>
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1420500}