The new reader is named H263Reader as it handles H.263 streams, but
MPEG-4 Part 2 streams are also intended to be handled. The reader's
output format MIME type is video/mp4v as the H.263 streams can be
decoded by decoders supporting this MIME type.

The implementation is based on the framework implementation for
extracting MPEG-4 video in MPEG-TS
(https://cs.android.com/android/platform/superproject/+/master:frameworks/av/media/libstagefright/mpeg2ts/ESQueue.cpp;l=1825;drc=86e363c1fac27302ca4ae33e73296f7797672995)
and is similar to the existing H262Reader.

Issue: #1603
Issue: #5107
PiperOrigin-RevId: 320565337