Fix ActionMailer config loading regression.

#d6dec7fcb6b8fddf8c170182d4fe64ecfc7b2261 added mail-preview feature that defined

`before: :set_autoload_paths` initializer, making all action_mailer initializers to be preloaded,
causing environment configs to be disregarded by action_mailer.

This moves `action_mailer.configure_mailer_previews` initializer to load as Railtie initializer to maintain correct behaviour.
Also since we affect `app.config.autoload_paths` don't freeze it in engine, but in AM initializer.

Fixes #13372