android-ndk: Rust bindings of the Android NDK

This is a work in progress at the moment.

android-ndk-sys contains the raw FFI bindings, pre-generated from NDK r20, and android-ndk provides a safe API over it.

Other helpful crates for Android:

• jni, JNI bindings for Rust
• android_logger and ndk-logger, Android backends for the log crate