CrOS login: Eliminate CaptivePortalStatus dependency in NetworkPortalDetectorMixin

NetworkPortalDetector::CaptivePortalStatus is being deprecated.
To facilitate this, use a local state enum in NetworkPortalDetectorMixin which
    can be translated to the correct state in one place when
    CaptivePortalStatus is deprecated.

BUG=331264838
TEST=browser_tests, unit_tests

Change-Id: I2f0f6514089ebe114b1e4ee3598509c55704f228
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5402892


Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Danila Kuzmin <dkuzmin@google.com>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282799}