Fix possibly bug-prone code structure.

access_token_fetcher_ field was initialized before other fields (eg.
metrics_). This means that ctors of metrics_ could be called after
LaunchFetcher, creating potentially bug-prone behavior that interacted
with metrics_ object before its contructor has been executed.

After refactor, remove unnecessary std::unique_ptr wrap of said field.

Make use of initializer-list and default-member-initializer consistent.

Bug: b:303031502
Change-Id: I87ef4af7597215480bbca0fb839b576ff9a4a08b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4907650
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Tomek Jurkiewicz <tju@google.com>
Cr-Commit-Position: refs/heads/main@{#1204000}