Use only the NDK as var/compiler on Android

As I wrote in #41677, I experimented on using only the NDK to build a few projects, especially Rust, and it worked well.

So, I'm proposing to create a new project (android-ndk) and to use it for stuff that needs native development.

The rest of android-toolchain can be moved to android-sdk, and be used only in the projects that actually need it.

It's also more in line with what projects such as application-services do.