Extract locale country code for variations on first run

[DJAndries]

Resolves brave/brave-browser#44599

Submitter Checklist:

• I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally:
  • npm run test -- brave_browser_tests, npm run test -- brave_unit_tests wiki
  • npm run presubmit wiki, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

Only test on Windows.

0. Find or create a study that is country specific and applies to the channel & version being tested.
1. Set the system locale to match one of the countries in the study.
2. Install & launch the browser for the first time, access brave://version, ensure study is enabled.
3. If desired, set the system locale to a country not included in the study, uninstall the browser.
4. Re-install & launch the browser. Ensure the study is not enabled.

[brave-builds]

Released in v1.78.49

[kjozwiak]

Verification PASSED on Win 11 x64 using the following build(s):

Brave | 1.78.49 Chromium: 134.0.6998.95 (Official Build) nightly (64-bit)
-- | --
Revision | 7edbd46f3dca287cb7238eda27536c9a84a2fea2
OS | Windows 11 Version 24H2 (Build 26100.3194)

Perquisites:

• ensured that the OS Language is set to the target locale
• ensure that the OS Region is set to the target locale
• ensure that you're using a IP address associated with the target locale (Canada/Mexico for this verification/check)

Test Case #1 - CA (Canada)

• downloaded the Nightly stub installer via https://brave.com/download-nightly
• ensured that 1.78.49 Chromium: 134.0.6998.95 was installed which includes the above change
• ensured that BraveSearchPromotionBannerStudy wasn't being applied and not visible under brave://version
  • country: ['DE','FR','US','AT','ES','MX','BR','AR','IN]` as per https://github.com/brave/bravevariations/blob/ee8ab1a3af4a4b8ab41d5d61db89838556e12ab3/studies/BraveSearchPromotionBannerStudy.json5#L62
• ensured that BraveSearchPromotionButtonStudy was being applied and was visible under brave://version
  • country: ['CA', 'GB'] as per https://github.com/brave/brave-variations/blob/ee8ab1a3af4a4b8ab41d5d61db89838556e12ab3/studies/BraveSearchPromotionButtonStudy.json5#L30

Example	Example	Example	Example

Test Case #1 - MX (Mexico)

• downloaded the Nightly stub installer via https://brave.com/download-nightly
• ensured that 1.78.49 Chromium: 134.0.6998.95 was installed which includes the above change
• ensured that BraveSearchPromotionButtonStudy wasn't being applied and not visible under brave://version
  • country: ['CA', 'GB'] as per https://github.com/brave/brave-variations/blob/ee8ab1a3af4a4b8ab41d5d61db89838556e12ab3/studies/BraveSearchPromotionButtonStudy.json5#L30
• ensured that BraveSearchPromotionBannerStudy was being applied and visible under brave://version
  • country: ['DE','FR','US','AT','ES','MX','BR','AR','IN]` as per https://github.com/brave/bravevariations/blob/ee8ab1a3af4a4b8ab41d5d61db89838556e12ab3/studies/BraveSearchPromotionBannerStudy.json5#L62

Example	Example	Example	Example