Split build based on destination store

Removes the Amazon in-app-purchasing-2.0.76.lib from Google apps, and the Google billing library from Amazon apps. Adds the appstore flavor to support building separate versions of the apps. See #30 and #29 for details.

API Tweaks and Amazon 100 IAP Fix

The 2.0.0 release with the big Google Billing update has been confirmed working for customers, so this release builds on that base with:

• Requests for > 100 IAP from Amazon will no longer crash, fixing #22
• A key is not required for init, as suggested by #10 & moved to #11
• All callbacks are on the main thread, as suggested by #10 & moved to #13
• Better handling for "this item already owned" was added, fixing #8
• The unused id parameter was removed from buy, subscribe and unsubscribe methods

Use Google Play Billing Library

• Removes ROT-13 encoding of the key from the library, so now you should pass an unencrypted key into the library and handle any key encryption (for protection against disassembly) directly in your app
• Requires a list of subscription keys be passed into the build() method
• Removes the old Google AIDL billing library and switches to the Google Play Billing 3.0 library.
• com.android.vending.BILLING is no longer needed in the Android.manifest file

In one of our apps, we also had to tweak the build.gradle file as follows:

implementation(rootProject.ext.general.inappPayments) {
        // we exclude this, because we already have the 'org.jetbrains:annotations-java5' library that has the same classes
        exclude group: "org.jetbrains", module: "annotations"
    }

Update All The Things

• Update to the latest Android SDKs and build tools
• Update to latest Amazon in-app purchase SDK
• Update Google subscription link

Note: It is likely that subscription support has been broken for a while, as it has been deprecated by Google.