Android Billing Library v2

[j3k0]

Rewrite for Android using the Billing Library V2.

To test it:

# 1: Uninstall the plugin (if already installed)
# 2: Install from github
cordova plugin add "https://github.com/j3k0/cordova-plugin-purchase.git#android_billing_library_v2"

[j3k0]

developerPayload is now supported + some related features.

Check developerPayload, applicationUsername and developerName here: https://github.com/j3k0/cordova-plugin-purchase/blob/39bd84a880bd19281ad5b8d720bc9123ccc13110/doc/api.md#storedeveloperpayload

[j3k0]

I've used some niceties of Java, which rise the cordova-android requirements to version 7 (it won't compile with cordova-android@6.4) However, since Google now require apps to support API Level 28, I don't think it makes sense to keep support for cordova-android < 8 anyway (cf https://cordova.apache.org/docs/en/latest/guide/platforms/android/#requirements-and-support).

What do you think?

[j3k0]

Merged into #911