Cleanup Google Sign-In permission toggles

[ShivanKaul]

Currently, the Google Sign-In pref in brave://settings is the same as the one in brave://settings/socialBlocking (which shows up if the feature flag for Google Sign-In is off) which is Allow use of third-party cookies for legacy Google Sign-In. This is not accurate, because really what the pref should indicate is that sites can ask for 3p cookie access.

Also, it's confusing that there's a toggle in brave://settings/content/googleSignIn which seems like a global toggle but is actually just the default toggle. We should get rid of that and only have one way of setting the pref. But then we need to remove the per-site pref from site details that shows up on (for e.g.) brave://settings/content/siteDetails?site=https%3A%2F%2Fgithub.com%2F and prevent the site permission from showing up in brave://settings/content. To do this we will need to restart the browser.

As an additional code-related cleanup, move privacy-related loadtime data booleans to browser/ui/webui/settings/brave_privacy_handler.cc instead of browser/ui/webui/brave_settings_ui.cc

In the future, we might want to sync the top-level pref with the site setting toggle.

To summarize:

1. Distinguish between "allow all" old pref text in brave://settings/socialBlocking and the new Google Sign-In feature pref text in brave://settings/privacy. Former is global exception, latter is per-site default-ask site permission.
2. If new feature pref is off, hide (a) site permission in brave://settings/content and (b) site details in brave://settings/content/siteDetails?site=xyz.
3. If feature pref changes, you should be prompted for a restart (needed in order to hide settings).
4. Clarify that what we're actually allowing/asking/blocking is access to third-party cookies in site permission text
5. Clarify exception text i.e. "Allow/Block these sites..."
6. Remove the default toggle on brave://settings/content/googleSignIn
7. Help link for new feature pref should go to a new wiki page, not the old one. Wiki page TBD.

[ShivanKaul]

Closing in favour of #28358