Allow for a per-locale default search engine (value comes from config)

[bsclifton]

Fixes #14647

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).
• 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. (Ask a Brave employee to help if you cannot access this document.)

Test Plan:

1. Grab source code
2. Either Delete or move out of the way your brave-development profile directory
3. Uncomment these two lines: https://github.com/bsclifton/browser-laptop/blob/bfdba0e5ec440f3f31bf9c87aec12557a0861173/js/constants/config.js#L49-L50
4. If you're not in the US, set your locale as English / US (note: this may not be trivial on macOS if you're using an Apple account; locale seems to be tied to iTunes region. I could be wrong)
5. Launch browser
6. Your default search engine should be GitHub (visit about:preferences#search)
7. Delete the brave-development profile directory
8. Set your locale as Spanish / Mexico
9. Launch browser
10. Your default search engine should be YouTube (visit about:preferences#search)

Automated test plan:

npm run unittest -- --grep="setDefaultSearchEngine"

Reviewer Checklist:

• Request a security/privacy review as needed if one was not already requested.

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[diracdeltas]

seems to work

[bsclifton]

master 5c46b3b
0.24.x a59631a
0.23.x 33d5246