Make Extractor/Mp4 Atom Parsing code reusable by external custom mp4 …

[colinkho]

…extractors

[icbaker]

Thanks for sending this. I'm going to submit some of this myself directly internally:

1. Move a couple of parsing methods from Atom to AtomParsers
2. Move Atom to lib-container and make it public (so it can in future be used by muxer)

Once these changes are submitted I'll let you know and you can rebase this change so it only has the AtomParsers, Sniffer and TrackSampleTable changes.

[icbaker]

The changes have been submitted:

• 9dfd72b
• 9d008da
• ee27334

The final change renamed Atom to Mp4Box for two reasons:

1. The 'box' terminology more closely matches the ISO 14496-12 spec.
2. Other MP4-specific files in androidx.media3.container are prefixed with Mp4 instead of being in an mp4 sub-package.

I suggest we rename AtomParsers to BoxParsers and keep it in the extractor.mp4 package. We don't need to also touch every call-site, because it makes the blast radius of the change too large.

I don't mind if you do that in this PR, or we can do it directly as an internal change.

[colinkho]

@icbaker Rebased to capture your commits and renamed to BoxParsers. Thank you!

edit: There was one commit from the pull that didn't pass CLA checks for some reason. Hence, I've rebased my commits to the top and force pushed for simplicity

[icbaker]

I'm going to send this for internal review now. You may see some more commits being added as I make changes in response to review feedback. Please refrain from pushing any more substantive changes as it will complicate the internal review - thanks!