[ads] Unify iOS kBraveNTPBrandedWallpaper

[tmancey]

Summary of Changes

This pull request fixes #8701

Requires https://github.com/brave/brave-core/pull/21751

Submitter Checklist:

• [ ] Unit Tests are updated to cover new or changed functionality
• [ ] User-facing strings use NSLocalizableString()
• [ ] New or updated UI has been tested across:
  • [ ] Light & dark mode
  • [ ] Different size classes (iPhone, landscape, iPad)
  • [ ] Different dynamic type sizes

Test Plan:

• Fresh install
• Launch browser
• Wait a few minutes for the seed to be fetched (or you can background/foreground the browser)
• Open *Settings > Version #.# (#.#.#.#) > View all version info > Active Variations**
• Check if Active Variations: list contains the enabled studies for iOS.
• If enabled studies for iOS are not listed in Active Variations: then terminate and start browser. Make sure that Active Variations: list contains the enabled studies for iOS.
• Open new tabs until an New Tab Takeover ad is shown
  • EXPECTED RESULT: New Tab Takeover ad should be shown on the nth BraveNTPBrandedWallpaper/initial_count_to_branded_wallpaper Griffin feature param
• Open new tabs until an New Tab Takeover ad is shown
  • EXPECTED RESULT: New Tab Takeover ad should be shown on the nth BraveNTPBrandedWallpaper/count_to_branded_wallpaper Griffin feature param

We should also test the default values if the Griffin feature does not exist. Default value is 2 for BraveNTPBrandedWallpaper/initial_count_to_branded_wallpaper and 3 for BraveNTPBrandedWallpaper/count_to_branded_wallpaper.

And that the position is persisted across browser restarts.

Screenshots:

Reviewer Checklist:

• [ ] Issues include necessary QA labels:
  • QA/(Yes|No)
  • 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 and pull request is assigned to a milestone (should happen at merge time).

[tmancey]

Tests are failing because we first need to merge brave-core changes with https://github.com/brave/brave-core/pull/21751