Show DDG newtab in Qwant region if Qwant is not used

[simonhong]

In Qwant region, DDG newtab should be used when user uses non-Qwant
search engine provider.
In non-Qwant region, user always see DDG newtab.

Close brave/brave-browser#1666

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:

1. Set region to France.
2. Start Browser with clean profile
3. Open private window and check non-ddg newtab is used
4. Change search engine setting
5. Then reopen private window and checks ddg newtab is used

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]

Getting clarification from @davidtemkin about how this should work. Since it's been changed a few times, would be great to have a solid set of requirements 😄 Hang tight!

[davidtemkin]

DDG should never be promoted / shown / defaulted on in Qwant regions. In Qwant regions, we default to Qwant for search in all types of windows, including private and private/tor and do not promote DDG.

[bsclifton]

Verified the behavior we would like with @davidtemkin and updated issue to have a proper test plan 😄 (which I manually verified). This should be good to go! 😄

[bsclifton]

master 0be354a
0.57.x 63157e0
0.56.x 64101fc