* add migration for social media blocking prefs
* move pref change registrar to AdBlockService
* change new preference path to avoid conflict
Settings UI pages automatically check for matching preferences
registered under the profile first. They only check under local state if
nothing was found registered under the profile prefs.
This means we can't use the same preference path after the migration.
* only attempt to migrate settings from regular profiles
The first profile to initialize is the System Profile, which will never
have any meaningful settings to migrate.
* use local state in the tests
* update BraveProfilePrefsBrowserTest
* register legacy prefs as part of `AdBlockPrefService`
* add comment in normal migration path
Anton Lazarev
0809524171