This is the callback that is invoked when an item is purchased, refunded, or canceled. It is the callback invoked in response to calling BillingService#requestPurchase(String). It may also be invoked asynchronously when a purchase is made on another device (if the purchase was for a Market-managed item), or if the purchase was refunded, or the charge was canceled. This handles the UI update. The database update is handled in {@link ResponseHandler#purchaseResponse(Context, PurchaseState, String, String, long)}.
