CCodec: don't release all references at flush

PipelineWatcher::flush clears all input buffer references, but
CCodecBufferChannel::flush should retain input buffer references
that are not returned yet.

Bug: 177462517
Test: atest CtsMediaTestCases -- --module-arg CtsMediaTestCases:size:small
Change-Id: I6c035df647b76077d979079e579d2300e7a25062