Correct field name is needed in order to have a valid promo code

[bsclifton]

Fixes brave/brave-browser#2344

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Windows
  • macOS
  • Linux
• Verified that these changes pass automated tests (npm test brave_unit_tests && npm test brave_browser_tests) on
  • Windows
  • macOS
  • Linux
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone

Test Plan:

• See STR in promo code is not imported from browser laptop brave-browser#2344
• Also see manual steps in my comment below

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

[bsclifton]

I manually verified this - here's how you can too 😄

1. Fresh browser-laptop profile (~/Library/Application Support/brave / %APPDATA%\brave)
2. Download latest 0.25.x release here https://github.com/brave/browser-laptop/releases/tag/v0.25.2dev (either the pkg for macOS or the 32/64 .exe for Windows)
3. Rename the executable like so: Brave-Browser-BRA941.pkg
4. Run the installer
5. Launch into Muon Brave (if installer didn't already do that)
6. Quit Muon Brave
7. Build this PR
8. npm run start
9. Run the importer; pick Stats for Brave (old)
10. When it runs, check out the console and see something like the following:

[38591:775:1203/215831.677078:INFO:brave_profile_writer.cc(328)] Setting kReferralPromoCode to "BRA941"
[38591:775:1203/215831.677224:INFO:brave_profile_writer.cc(336)] Setting kReferralDownloadID to "d8a77779-c440-4446-a9ec-16f4095e703f"

11. If you see similar to the above, you're good to go! 😄

[bsclifton]

master 3f0f487
0.59.x e455c22
0.58.x efb24a5
0.57.x 7893b94