fix(privacy): Fix Shred on app close

[StephenHeaps]

• Apple kills the app before we have a chance to shred. Update the Shred on App Close logic to not restore tabs for sites with 'App Close' preference set, and then shred the sites after tab restore.
• Fix bug causing tabs closed before app is quit to not be Shred on app close.
• Fix feature flag not correctly wrapping Shed on App Close logic.

Resolves brave/brave-browser#40484

Submitter Checklist:

• I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally:
  • npm run test -- brave_browser_tests, npm run test -- brave_unit_tests wiki
  • npm run presubmit wiki, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

1. Enable Shred feature flag (#brave-shred) in brave://flags
   • Note: 3rd party caches will remain in Manage Website Data without the #brave-shred-cache-data flag enabled
2. In Shields settings, update 'Auto Shred' preference to 'App Close'
3. Open a few websites in different tabs
4. Close one of the tabs (before quitting the app)
   • This will be to verify site data from tabs closed before app is quit are also Shred.
5. Fully quit the app from iOS app switcher and re-launch
6. Verify All tabs are closed
   • Note: A site-specific Shred preference can be applied Shields panel when visiting a site
7. Open 'Settings' -> 'Shields & Privacy' -> 'Manage Site Data'
8. Verify all 1st party site data is removed
9. Open 'History'
10. Verify site(s) are removed from history
11. Open a few websites in different tabs
12. In one of the tabs, open shields panel, tap 'Advanced controls' and tap 'Shred Site Data'.
13. Change site preference to 'Never'
14. Fully quit the app from iOS app switcher and re-launch
15. Verify all tabs are closed except the site manually changed to 'Never'
16. Open 'Settings' -> 'Shields & Privacy' -> 'Manage Site Data'
17. Verify all 1st party site data is removed except the site manually changed to 'Never'
18. Open 'History'
19. Verify site(s) are removed from history
20. Do a quick pass with 'Auto Shred' set to 'Site Tabs Closed' (Shred is delayed 30 seconds after tab close in this case)

[brave-builds]

Released in v1.73.23