[SAML] Add SamlRedirect to Gaia paths and improve tests

Handle "samlredirect" Gaia path the same way as other Gaia paths used by
ChromeOS and start checking it in some SAML tests.

Details:
- GaiaUrls now knows about "samlredirect" Gaia path
- kSamlRedirect is added to WizardContext::GaiaPath enum and it is now
respected by GaiaScreen and GaiaScreenHandler
- "samlredirect" is now passed as a "gaiaPath" from
GaiaScreenHandler/LockScreenHandler to JS code. In particular,
"samlredirect" is no longer hardcoded in authenticator.js
- Implementation of "Enter Google Account Info" button had to be
changed: previously it relied on the fact that "gaiaPath" pointed to a
default Gaia sign-in page, now there is a separate "fallbackGaiaPath"
parameter passed from C++ for this button (both on the login screen and
on the lock screen)
- Several SAML tests now verify the Gaia path used by the Gaia screen.
- New test SamlReauthWithSamlRedirect tests reauth flow for SAML user
with LoginAuthenticationBehavior policy set to navigate directly to SAML
page
- SAMLPolicyTest methods which update policies now correctly wait for
corresponding CrosSettings to update. This restores the behavior which
was mistakenly removed during refactoring in
https://crrev.com/c/4489824.

Bug: b:309148088, b:287922689
Change-Id: Icac9abae601802a31453667b517d225def1e612a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5007622
Commit-Queue: Andrey Davydov <andreydav@google.com>
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Reviewed-by: Mohammed Abdon <mohammedabdon@chromium.org>
Reviewed-by: Danila Kuzmin <dkuzmin@google.com>
Cr-Commit-Position: refs/heads/main@{#1221566}