[camera_android_camerax] Fix NV21 buffer over-allocation and optimize per-frame conversion

[Mairramer]

planesToNV21 was allocating the NV21 buffer based on yBuffer.remaining() instead of width * height. On some devices, the Y plane is backed by a larger shared direct buffer, which caused the output buffer to be over-allocated and the resulting NV21 size to be incorrect.

This PR fixes that by using width * height as the canonical Y size. It also removes a secondary inefficiency in the padded-row path: the old code allocated a temporary row array per row and copied it via System.arraycopy; the new code reads directly into the output buffer with yBuffer.get(outBuffer, position, width), eliminating both the allocation and the copy.

A new overload planesToNV21(planes, width, height, byte[] outBuffer) is added, which accepts a caller-owned output buffer. ImageProxyUtilsProxyApi.getNv21Buffer now allocates the destination byte[] once and passes it directly to this overload, so the frame data is written in-place with no intermediate buffer and no extra copy.

Fixes #flutter/flutter190299

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[Mairramer]

/gemini review

[gemini-code-assist]

Code Review

This pull request optimizes NV21 buffer allocation and conversion in the camera_android_camerax package by passing the ImageProxy directly to getNv21Buffer and limiting plane buffers to prevent over-allocation. Feedback suggests avoiding the stateful nv21Buffer cache in ImageProxyUtilsProxyApi by directly allocating the destination array to eliminate an extra copy and maintain thread safety. Additionally, it is recommended to avoid temporary row array allocations in ImageProxyUtils.planesToNV21 by reading directly into the output buffer.

[Mairramer]

/gemini review

[gemini-code-assist]

Code Review

This pull request modifies the camera_android_camerax package to support writing NV21 data directly into a caller-provided buffer, updates ImageProxyProxyApi to limit plane buffers to their valid bytes, and adapts the Pigeon interface and tests. Feedback highlights a potential IllegalArgumentException in ImageProxyUtils due to unconditional buffer position adjustments on the last row of the Y plane, and notes that ImageProxyUtilsProxyApi still allocates a new byte array on every call instead of caching and reusing the buffer to reduce GC pressure.