Fix bugs with large initial audio discard padding

There were two issues with our current handline of multi-packet
audio discard values on the first packet:
* Subsequent packets marked for full discard by AV_PKT_FLAG_DISCARD
 would have their timestamp set to kInfiniteDuration - pts. Which
 sent duration calculations to the moon.
* As initially implemented we only handled the case where the audio
 discarded had a timestamp >= 0. The file in the bug applies multi
 packet discard to audio with timestamps <= 0.

Unfortunately I haven't been able to generate a test file to add
for this behavior. The user provided clip is too large and can't
be truncated or remuxed without losing its special properties.

Bug: 435684931
Change-Id: I5c12825338c8a4b36070733de5f33f5f2b3f6303
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6838830
Reviewed-by: Eugene Zemtsov <eugene@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1500887}