SDK / exoplayer

As highlighted by the ref'd issue, we can end up with
memory leaks if Loadable.load implementations take a long
time to return upon cancelation. This change cuts off one
of the two problematic reference chains.

This doesn't do much about the ref'd issue, since there's
a second reference chain that's much harder to deal with:
Thread->LoadTask->loadable. But since it's easy just to
cut this one off, I figure it makes sense to do so.

Issue: #4249

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=198735386