AndroidScheduler.java

Initial spike for AndroidScheduler.java.

Been using this for quite some time. But I don't like the concept using calling `observer.observeOn(AndroidScheduler.getInstance())` as it sort of puts Android specific stuffs. Should RxJava support some concept of `Schedulers.mainScheduler()`? We can then set the default main schedulers using `Schedulers.setMainScheduler(AndroidScheduler.getInstance())` when app starts.