Fix #5636: Invalidate NTP layout when sections change to fix News layout

[kylehickinson]

On first launches default favourites are added async which cause FRCs to fire after NTP is already laid out, which trigger sectionDidChange. When this happens we were reloading the changed sections without animation but this would sometimes cause layout issues with sections not reloaded like Brave News.

Summary of Changes

This pull request fixes #5636

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

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).