[M152] Add flag to force US country code for AIM eligibility

Original change's description:
> Add flag to force US country code for AIM eligibility
>
> Introduce the `kAimEligibilityForceUsCountryCode` feature flag to allow
> forcing US-based country parameters during AIM eligibility requests.
>
> When this feature flag is enabled:
> * `gl=us` is appended to set the Google geolocation query parameter to
>   the United States.
> * `client_country=us` is appended to pass a forced US country code to
>   the server eligibility service, overriding the default code fetched
>   from variations.
>
> These parameters ensure consistent evaluation of US-specific AIM
> eligibility logic and allow testing US eligibility behavior across
> different client regions.
>
> Change-Id: I46560e135878bad641895ddb92c82ae78788347d
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8165142
> Reviewed-by: Duncan Mercer <mercerd@google.com>
> Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
> Commit-Queue: Diana OuYang <dianaou@google.com>
> Cr-Commit-Position: refs/heads/main@{#1670416}

(cherry picked from commit 3d222a77f7fc05676104f3672f8e0e2bd71dd6cb)

Bug: 541298244
Change-Id: I46560e135878bad641895ddb92c82ae78788347d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8183766
Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Juliet Lévesque Knighton <julietlevesque@google.com>
Cr-Commit-Position: refs/branch-heads/7977@{#526}
Cr-Branched-From: b7fe14017379ddffae396d944fb8b59a5896c261-refs/heads/main@{#1669021}