Fix #3177: Update pre-populated search engine list

[iccub]

This also replaces Yahoo with Ecosia on search onboarding screen.

Summary of Changes

This pull request fixes #3177

Submitter Checklist:

• Unit Tests are updated to cover new or changed functionality
• User-facing strings use NSLocalizableString()

Test Plan:

1. New user

• Set region with Ecosia enabled, United States for example
• Fresh install the app
• Verify that Ecosia is one of search options showed during onboarding
• Verify that Yahoo is not listed as one of search options
• Select it, make sure search queries use this search engine
• Verify that correct search parameters are passed, you can refer to https://github.com/brave/brave-browser/wiki/Search-engine-integrations

2. Existing user

• Install previous app build, select some search engine
• Update to build with Ecosia, make sure eligible region(like US) is set
• Go to Brave Settings-> search engines
• Verify that Ecosia is on the list, select it
• Verify that correct search parameters are passed

3. Existing Yahoo user

• Install previous app build, select Yahoo as default search engine
• Update to build with Ecosia, make sure eligible region(like US) is set
• Verify that default search is still Yahoo

Screenshots:

Reviewer Checklist:

• Issues include necessary QA labels:
  • QA/(Yes|No)
  • release-notes/(include|exclude)
  • bug / enhancement
• Necessary security reviews have taken place.
• Adequate unit test coverage exists to prevent regressions.
• Adequate test plan exists for QA to validate (if applicable).
• Issue is assigned to a milestone (should happen at merge time).

[bsclifton]

Changes look great to me! We'll keep Yahoo as an available engine (not in onboarding of course) until we can support having it migrated off (maybe after merging OpenSearch code)

[iccub]

@bsclifton yes, i tried to move yahoo out, but found no simple way to do it, we will revisit it later

[soner-yuksel]

This property is temporary right until we remove the yahoo engine totally ?

[iccub]

I think we can keep it, if we add a lot of predefined search engines in the future we can choose which of them to show during onboarding, to avoid having a long list of search engines