[iOS]AccountPickerCoordinator owns the AddAccountCoordinator

This not only reduce code duplication. But furthermore, avoids the
need of having a completion callback calling another completion
callback, which will help transforming the AddAccountSigninCoordinator
so that it uses a delegate instead of a completion block.

Bug: 381444097
Change-Id: I7d19a4eb4a18b84c44670bcd2fb6e05025aa1808
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6549277
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Auto-Submit: Arthur Milchior <arthurmilchior@chromium.org>
Commit-Queue: Arthur Milchior <arthurmilchior@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1463373}