[camera_android_camerax] Fix NV21 Format

[camsim99]

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

1. The NV-21 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.

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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?