What's the minimum Android API level version we want to support?

Rust 1.68 updated the minimum supported Android NDK to r25 (https://blog.rust-lang.org/2023/01/09/android-ndk-update-r25.html), which begs the question: what is the minimum Android API level we want to support in this project? We should probably agree on and document one somewhere; if it's lower than 19 (KitKat), we'll need to make sure to build with Rust <1.68 going forward.

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