Add android support for the App feature

[iota-xSK]

Android support would be very useful as there are very few crates that have good android support at the moment. It wouldn't be that hard to add either, just some way to provide the AndroidApp to the EventLoop through the with_android_app() method on the builder. Everything else should work as normal once the event loop is initialized. I tried getting this working but so far (today) I wasn't successful. Thoughts?