Fix 6804: Avoid the OnPreferenceChanged callback when preferences are changed by CookiePrefService

[jumde]

Fix brave/brave-browser#6804

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Android
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that these changes pass automated tests (unit, browser, security tests) on
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that all lint errors/warnings are resolved (npm run lint)
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone
• Public documentation has been updated as necessary. 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-Protection-Mode
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-compatibility-issues-with-tracking-protection
  • https://github.com/brave/brave-browser/wiki/QA-Guide

Test Plan:

1. Navigate to brave://settings/shields
2. Toggle Cookie settings from Only block only cross-site cookies to Block All Cookies
3. Refresh and verify the setting remains the same
4. Toggle the setting to Block-Cross site cookies and refresh
5. Verify the setting remains the same
6. Navigate to https://www.whatismybrowser.com/detect/are-third-party-cookies-enabled and verify that third party cookies are blocked
7. Toggle the setting to Block all cookies -> Allow all cookies
8. Navigate to https://www.whatismybrowser.com/detect/are-third-party-cookies-enabled and verify that all cookies are allowed

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

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.

[mihaiplesa]

Has passed for Android, iOS, Linux, Windows so applied skip labels and will need to be re-built only on macOS (unless no new changes pushed).

[iefremov]

Can we please write a detailed comment explaining this approach? It is sort of non-trivial to understand