Example with wrapper module (pay-lib) for using Google Pay Api in one-step

This example is an attempt to make a simple and more refined example of utilization the Google Pay API than in the google-pay/android-quickstart project (unfortunately, a lot of unstructured constants and json-objects (that can be encapsulated) might confuse a beginner).

This example is suitable for digital purchases when you decide to use your bank's processing. For start payment you should

    /* get this data from your bank */
    val merchantData = MerchantData(
        merchantName = "example",
        gateway = "example",
        gatewayMerchantId = "example"
    )

    /* data about current item payment */
    val paymentRequest = PaymentRequest(
        itemToBuy.name, itemToBuy.description, itemToBuy.priceCents,
        "USD", "US"
    )

    /* start Activity and enjoy */
    PaymentActivity.openPaymentActivity(
                    applicationContext,
                    merchantData,
                    paymentRequest,
                    isTest
                )