[FEATURE REQUEST] Google Play Store Billing

[pozylon]

Introduction
We currently support Apple In App Purchase for one-time and subscription payments. This is a unique feature making Unchained a cool system for native Apps or Mobile Games. But by only having the Apple way, it's kind of half-complete and not really ready to market it.

Describe the solution you'd like
Build a Google Play Store Billing Payment Plugin analog to the Apple In-App-Store Payment Plugin.
https://developer.android.com/google/play/billing