[ios] Change ChangeProfileContinuation to a callback

The protocol has a single method, and the method should only be
called once so change the type to a callback and convert client
code.

This make the implementation slightly simpler.

Bug: none
Change-Id: Icca3d165f62488e3a6cfc263e5f4bff9c7fc224f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6148823
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1404743}