Replace deprecated use of AVCodecContext::reordered_opaque

We can use the AV_CODEC_FLAG_COPY_OPAQUE flag on the codec context
now to trigger timestamp propagation.

Bug: 330573128
Change-Id: I6bc57241a35ab5283742aad8d42acb4dc5e85858
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5384308


Commit-Queue: Ted (Chromium) Meyer <tmathmeyer@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1277051}