API: Avoid unstubbing User for language pref when not needed

API: Avoid unstubbing User for language pref when not needed

It's fairly common that the API doesn't need to load the user
preferences, except to implement the unfortunate default uselang=user.

So let's move the handling of uselang=user to
RequestContext::getLanguage(), and have the API just assume that its
parent context will eventually fall back to that for uselang=user.

Bug: T85635
Change-Id: I947348d87b31808d331055dac6feb0cc2e1dd15d