Currently, input/output buffers can 'leak' if they are dequeued then the decoder
is flush()ed. This happens if an input buffer is dequeued then a discontinuity
is read, or an output buffer is dequeued and is early. If this happens several
times, no more buffers are available.
This change makes flush() work like MediaCodec: it returns all dequeued
input/output buffers to the codec. Keeping the behavior in line with MediaCodec
might make it easier to factor out a common decoder interface in the future.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=109054178
