SDK / exoplayer

In V1 we received complaints that Renderer was too locked down.
In particular, it wasn't possible to implement a Renderer that
wrapped another Renderer, since most of the methods were package
private and final (e.g. enable).

This change defines Renderer as a proper interface, removing
this limitation. A method-reordering-only change will follow
this one, to get things into a nice consistent/sane order in
each of the Renderer classes.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=127527995