Split Android code out to also support Java

## 🚀 Feature Proposal

Make it possible to use this library for normal Java apps.

The dependency on Android in this project seems fairly minimal so it seems like it should be possible to abstract it and offer two implementations.

## Motivation

The codebase I deal with at work has a mix of web and Java desktop apps. We localise into 11 languages and currently use ICU plural formats on the Java side, which developers have a hard time dealing with.

I believe that the i18next format might make it easier to do the right thing with plurals.

## Example

Similar to what's documented for the Android version, except that overloads taking `Locale` would be more convenient.