Add a hook into LanguageConverter#getPreferredVariant() to allow extensions to pull...

Add a hook into LanguageConverter#getPreferredVariant() to allow extensions to pull the desired variant from cookies (or other such source)

Example implementation using this hook: wikiHow's ChineseVariantSelector
extension, installed on zh.wikihow.com, which uses cookies to store the
preferred language variant, allowing anonymous users to change the
language variant without registering/logging in.

Change-Id: I5295a26578b45a8d51f2b7550938088fec18404f