[camera_android_camerax] Fix NV21 Format

[camsim99]

When NV21 image format is requested for streaming images, this PR ensures that:

1. The NV21-compatible (but actually YUV_420_888) three planes per image are converted to a single NV21 plane (Y + interleaved VU planes).
2. The single CameraImagePlane created has overridden raw and camera image format NV21.*

This should make this package compatible with google_ml_kit_flutter 🤞 I tested this change with the Barcode scanner example. Fixes flutter/flutter#174923.

*The conversion will fail if the image is actually not NV21 compatible for some reason, so this should never be a false positive.

Note: Uses code inspired by googlesamples/mlkit.

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[camsim99]

This is directly inspired by this camera_android code that came from Google MLKit. I made some changes the methods to (1) take in the objects I want and (2) for planesToNV21, I worked off of yuv420ThreePlanesToNV21 but deleted lots of logic and added an exception. So, I'm not sure what to do here....

Should I also link to https://github.com/googlesamples/mlkit/blob/master/android/vision-quickstart/app/src/main/java/com/google/mlkit/vision/demo/BitmapUtils.java and call it a day or are there more license implications?

[camsim99]

@stuartmorgan-g Do you know what we should do in cases like this by chance?

[stuartmorgan-g]

Should I also link to https://github.com/googlesamples/mlkit/blob/master/android/vision-quickstart/app/src/main/java/com/google/mlkit/vision/demo/BitmapUtils.java and call it a day or are there more license implications?

Third-party code cannot be handled that way; the camera_android code shouldn't have landed like that (unless it had OSPO approval), so it should definitely not be replicated, and the old code should be fixed.

The standard way to handle this would be:

• Land it in a third_party directory, with the correct license.
• Include that license in the package LICENSE file.

However, because it is specifically Google that has the copyright on that code, it may be that there are simpler options. I would reach out to OSPO and ask how this should be handled.

[camsim99]

Thanks for the guidance! Reached out to OSPO and they let me know that if our code is derived from code authored by Googlers or contributed to a Google-managed open source project under the terms of Google's contributor license agreement, then I can treat the code as though we wrote it ourselves.

Because the BitmapUtils.java code comes from mlkit/android/vision-quickstart that is licensed under the aforementioned agreement, I think I am okay to use it without reference to the original code. I will go ahead and make edits to accommodate this fact and also make a follow-up PR to remove the reference from camera_android.

[camsim99]

Just FYI: I did send an email back with my intentions just to sanity check my decision here!

Edit: They confirmed this is the right move 👍

[stuartmorgan-g]

Updating brach to pick up the license format check change, since this adds new files. The new files will need to be updated to match the new header boilerplate.