[ios][auth-on-entry2] Animate password settings presentation

Polishing CL. Animate the presentation of password settings when auth
is required by inserting the auth view controller in the navigation
stack before the stack itself is presented. This way we can present
password settings with animation without revealing its content until
successful auth.

Bug: 1462419
Change-Id: I2157c0983f3ed11e7ab6035801b49754e8431761
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4935301
Commit-Queue: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Commit-Queue: Ernesto Izquierdo Clua <eic@google.com>
Cr-Commit-Position: refs/heads/main@{#1209658}