Fix #8673: Move NTP P3A helper creation outside of NTP

[kylehickinson]

There was a chance that a user would simply never load a NTP during their session which means that P3A rotations wouldn't trigger new NTP-SI reports each day. This report was supposed to occur each rotation, not specifically per NTP view.

Summary of Changes

This pull request fixes #8673

Submitter Checklist:

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

Test Plan:

1. Prefix: Visit debug settings > BraveCore Switches and set express rotation interval of some short amount (and optionally do not randomize)
2. Load app as usual, view a NTP-SI and verify it reports correctly
3. Load a website in the current tab and kill the app
4. Re-open the app and do not visit any NTP (including non-SI versions), verify NTP-SI data is still reported correctly

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).