Round early seeks in MP3 files down to zero

Seeking in MP3s is not precise due to our usage of AVFMT_FLAG_FAST_SEEK;
which works by looking for 3 contiguous MP3 packets since MP3 headers
are simple enough they can occur by random chance in a byte stream.

Bytes early in the stream often look enough like an MP3 header to trip
up ffmpeg's seeking logic. As a workaround, round seeks within the first
frame to zero. The audio renderer will trim off the excess later.

Technically these issues can happen anywhere in the stream since the MP3
header is not complex enough to avoid it happening by chance. We could
also fix this by not using AVFMT_FLAG_FAST_SEEK, but that hurts seeking
performance on large files too much.

R=tguilbert

Fixed: 415092041
Change-Id: Id96d7cc575f714dc5aac2975b60826fc01bc16e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6618608
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1469495}