Refactor exclusions in ShouldSanitizePreference() to the pref list.

This is second issue related to MR !765 (closed) which requires uplifting changes to upstream as per Tom's suggestion:

   tjr | my suggestion would be to make a new override list, move these prefs
       | (https://searchfox.org/mozilla-central/source/modules/libpref/Preferences.cpp#6137-6143) into it, along with your pref.

Please refer to original MR for details. I'll update code comments and post it here.

/cc @pierov @tjr