Revert "Refactoring: Make AuthSessionStorage::Peek result const"

This reverts commit 0d207fc8.

Reason for revert: b/296195532, CHECK in GetAuthFactorsConfiguration
fails.

Original change's description:
> Refactoring: Make AuthSessionStorage::Peek result const
>
> Bug: b/238606050
> Change-Id: I6f9c8c1b7e9b9dd15808aaaa6fc106d7ecf3c64d
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4753034


> Reviewed-by: Toni Barzic <tbarzic@chromium.org>
> Commit-Queue: Denis Kuznetsov <antrim@chromium.org>
> Auto-Submit: Denis Kuznetsov <antrim@chromium.org>
> Reviewed-by: Elie Maamari <emaamari@google.com>
> Cr-Commit-Position: refs/heads/main@{#1180380}

Bug: b/238606050, b/296195532
Change-Id: I1e8869253a85b0eaec4d778bc50c0695139f999e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4783591


Reviewed-by: Martin Bidlingmaier <mbid@google.com>
Auto-Submit: Anastasiia N <anastasiian@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Commit-Queue: Achuith Bhandarkar <achuith@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1184838}