[iOS] Cleanup profile switching in AuthenticationFlow+Performer

AuthenticationFlow calls AuthenticationFlowPerformer to execute all of
the involved actions. AuthenticationFlowPerformer then calls back into
the flow via one of the `did..` methods on its delegate - except for
`switchToProfileWithIdentity`, which (before this CL) instead takes a
callback.
This CL changes `switchToProfileWithIdentity` to instead function like
all of the other performer methods, for consistency.

Bug: 375605482
Change-Id: Idd68980351f162e059cb3df7ea5b4d1be838940e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6357593
Commit-Queue: Arthur Milchior <arthurmilchior@chromium.org>
Reviewed-by: Arthur Milchior <arthurmilchior@chromium.org>
Auto-Submit: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1433449}