SDK / exoplayer

I think it's likely we'll revert back to discarding media
in sync with the playback position for ExtractorMediaSource
and HlsMediaSource too, where the tracks are muxed with ones
we're requesting anyway.

Note: discardBuffer is named as it is because it'll also be
used to discard for enabled tracks soon, as a result of the
remaining TODO in ChunkSampleStream. For enabled tracks the
discard will also be conditional on the samples having been
consumed, obviously.

Issue: #2362
Issue: #2176

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