For decoder reuse, we want disable() to flush the decoder. However,
if the flush needs to release the decoder for some reason, it seems
non-ideal to immediately re-initialize it. Re-initialization can
also throw ExoPlaybackException, which we don't want for disabling.
This change allows a variant of flush that wont re-initialize the
decoder if it has to be released, which will be used from disable().
Issue: #2826
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=216834862
842f622d29