Add Android native (ndk) implementation

[russhwolf]

There's currently no implementation shipping with the library that runs on Android native targets. In principle it should be possible to build one that talks to SharedPreferences via JNI. Is this something that people would find useful?

[russhwolf]

I have a prototype of this which seems to work well, though it caches JNIEnv in a way that I think will make it fail in multithreaded scenarios and I don't know a good way to avoid that while conforming to the Settings API. It's also quite complex to build a test harness around since there's not really anything out-of-the-box there for the androidNative targets, but I do have a mostly working solution there.

In any event, I know a lot more about JNI now than I did a few months ago.