aom: generate codec extradata to ensure correct muxing

Quoting @robux from #21207 (closed):

We should also generate the codec extradata from libaom as done in https://aomedia.googlesource.com/aom/+/ec0833fc9df73c0dc283e803881a121c395c4c5e So it can be muxed properly in MP4 and Matroska. Right now files produced are technically invalid even though they may play.

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