[Android] Flatten data structure like dict in arg types

[kirklandsign]

🚀 The feature, motivation and pitch

Per @JacobSzwejbka in python, basically everything gets flattened by a pytree lib, so on client like Android we would need to flatten as well. We need to support it to enable use case like #8665 (comment)

Alternatives

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Additional context

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RFC (Optional)

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cc @cbilgin