[ios] Fix block capturing pointer to C++ objects

The implementation of CredentialProviderMigratorAppAgent was using
a block that captured a pointer to multiple C++ objects (profile &
passkeyModel).

Simplify the implementation by using an helper function (easier to
use with base::BindOnce(...) without risking accidentally capturing
variable by a block) and an helper method.

While doing that, split the code performing the migration for all
profiles and for a specific passkey model into two methods and move
the shared logic into an helper method.

Fixed: 408430684
Change-Id: Id2537ed2af9e64eb88ad8d4fea80dd11838655be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6432433
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1443402}