Public interfaces nested in internal Kotlin types not marked as internal

[mmarinchenko]

Context: xamarin/GooglePlayServicesComponents#454 (comment)

If I'm not mistaken this issue is similar to #682 but is related to nested interfaces.

[mmarinchenko]

@jpobst, @jonpryor Thanks guys!

[jpobst]

This fix had to be reverted because it was implemented incorrectly.

#854