Support a callback on subscription renewal

[akshetpandey]

Before this change, the callback would have come before the JS would have had a time to initialize, this change makes it so that once the JS is initialized, the app can request for SKPayment events.

Anyone who wants to use this, will need add some code like and then call _setQueuedPurchaseHandler once they are ready to start accepting SKPayment events. Existing behavior doesn't change.

_setQueuedPurchaseHandler () {
  InAppUtils.setQueuedPurchaseHandler((error, response) => {
    try {
      // Handle error/response
    } finally {
      _setQueuedPurchaseHandler()
    }
}