SDK / exoplayer

Having moved HLS to use single sample queues per track, these
classes have become relatively similar. This CL aligns the two
to make this more obvious. It remains unclear whether it'll
ever be sensible to merge them; there are still some niggly
complications for HLS.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=120429618