Android implementation updates

[litinskii]

This is the work of #174 with updated readme.

For most question changes are described at #174.

just added some notes to readme
main question about allowedPaymentMethods and how to generate publicKey in section below

const METHOD_DATA = [{
  supportedMethods: ['android-pay'],
  data: {
    supportedNetworks: ['visa', 'mastercard', 'amex'],
    // https://developers.google.com/android/reference/com/google/android/gms/wallet/WalletConstants.html 
    // PAYMENT_METHOD_CARD and PAYMENT_METHOD_TOKENIZED_CARD.. or [1, 2]
    allowedPaymentMethods: [1, 2], 
    currencyCode: 'USD',
    environment: 'TEST', // defaults to production
    paymentMethodTokenizationParameters: {
      tokenizationType: 'NETWORK_TOKEN', // or GATEWAY_TOKEN
      parameters: {
        publicKey: 'your-pubic-key' // https://developers.google.com/pay/api/android/guides/resources/payment-data-cryptography#using-openssl
        //  if use GATEWAY_TOKEN
        //  gateway: 'your gateway',
        //  gatewayMerchantId: 'your gatewayMerchantId',
        //  merchantName: 'your merchantName',
      }
    }
  }
}];

[litinskii]

Update tokenization

const METHOD_DATA = [{
  supportedMethods: ['android-pay'],
  data: {
    supportedNetworks: ['visa', 'mastercard', 'amex'],
    // https://developers.google.com/android/reference/com/google/android/gms/wallet/WalletConstants.html 
    // PAYMENT_METHOD_CARD and PAYMENT_METHOD_TOKENIZED_CARD.. or [1, 2]
    allowedPaymentMethods: [1, 2], 
    currencyCode: 'USD',
    environment: 'TEST', // defaults to production
    paymentMethodTokenizationParameters: {
      tokenizationType: 'PAYMENT_METHOD_TOKENIZATION_TYPE_DIRECT', // or PAYMENT_METHOD_TOKENIZATION_TYPE_PAYMENT_GATEWAY 
      parameters: {
        publicKey: 'your-pubic-key' // https://developers.google.com/pay/api/android/guides/resources/payment-data-cryptography#using-openssl
        //  if use PAYMENT_METHOD_TOKENIZATION_TYPE_PAYMENT_GATEWAY
        //  gateway: 'your gateway',
        //  gatewayMerchantId: 'your gatewayMerchantId',
        //  merchantName: 'your merchantName',
      }
    }
  }
}];

[litinskii]

Add intermediateSigningKey for G pay integration.

intermediateSigningKey: {
    signatures: [array],
    signedKey: string
}