Fix #7876: Replace Rewards ads usage P3A question

[kylehickinson]

This removes the Brave.Rewards.AdsEnabledDuration question and answers the new Brave.Rewards.AdTypesEnabled question

Summary of Changes

This pull request fixes #7876

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 Installs should no longer report Brave.Rewards.AdsEnabledDuration
• Verify that Brave.Rewards.AdTypesEnabled sends correct answers for ads/NTP-SI settings

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).