Installing NDK automatically

Right now when appropriate NDK is not installed the build will fail with
```
Target native_assets failed: Error: Android NDK Clang could not be found.
```

IIRC gradle will install ndk automatically for `externalNativeBuild` projects. We do the same thing in [cargokit](https://github.com/irondash/cargokit/blob/97668e95b40a09aecd06bef8c627d38b5f699fd8/build_tool/lib/src/android_environment.dart#L60) (glue used to build rust code for Flutter plugins). That results in a smoother experience compared to failing the build. I'm currently looking into reimplementing cargokit in terms of native assets and this would be a bit of a downgrade.

This does raise question whether running build should be allowed to install NDK (or other toolchains for that matter). At least for gradle/NDK nobody seems to object.

Longer term it would be nice to have an option to install toolchains as part of build. When user has `rustup` installed, cargokit will currently install missing toolchains and targets. But this means that build is modifying files outside of build folder (or pub cache). Which does feels a little bit iffy. It would be great if a hypothetical `native_toolchain_rust` could install and manage rust toolchain, possibly stored in `.pub_cache` so that it can get reused between project. Or maybe this could be done during `pub get` through a hook, but at that point we don't know the target architecture so for rust for example we'd need to install all targets, which might be an overkill.