This makes the class available to custom MP4-parsing implementations,
while also allowing it to be used by `muxer` in future.
'Box' is the term used throughout the ISO 14496-12 spec, while the
'Atom' nomenclature was used in an earlier form of the spec
(Quicktime).
This change moves it from `extractor.mp4.Atom` to `container.Mp4Box`,
to be consistent with existing MP4-specific types in the `container`
module like `Mp4TimestampData`.
PiperOrigin-RevId: 663274752
ee27334f06