Fix Android crash computing brand

Fixed a crash on Android in some environments when computing brand and brand images.

Add UnionPay as a CardBrand. (It was previously successfully charging but returning 'Unknown' as the brand.)

Fix several crashes and brand handling

Fixes several crashes:

• Crash on iOS in createPaymentMethod if the card has address fields. (#119)
• Crash on iOS in paymentContextDidUpdateShippingAddressCompletion if no shipping methods provided.
• Crash on Android in StripeConfig if requiredShippingAddressFields is null.

Improves handling of brands and images on Android. (Latest SDK "brand" values changed.)

Make Credit Card Images available

Add method to Card that allows credit card images to be retrieved from the Stripe SDK.

Update to Stripe Android SDK v12

Updates the Stripe Android SDK to v12.0.1.

No new functionality in this release.

Add support for createSource().

Add support for createSource() in the custom integration since createToken() is deprecated by Stripe.

Fix Android Crash With Prefilled Address

Fixes a problem where creating a PaymentSession with prefilledAddress crashes on Android.

Re-read StripeConfig when new PaymentSession created

When a new StripePaymentSession is created, settings from StripeConfig are re-read and applied to the new session.

Previous behavior was to make StripeConfig read-only once the first StripePaymentSession had been created.

Fix XCode 11.3+ build problem

Fixes #97

Fixes problem building when using XCode versions 11.3 or 11.4.

Fixed by upgrading to Stripe iOS SDK v19.0.
This required changing the minimum supported iOS platform version from v9 to v10.

Although the API is compatible with v6.2.1, the minor version was updated because of the change in minimum supported iOS platform version.

Fix Vue build problem

Fixes #87
Was referencing tns-core-modules/utils/utils.ios instead of tns-core-modules/utils/utils.

Fix NS v6 build problems

Fixes problems with building on NS v6.

If client uses any of the package enums, the WebPack transpileOnly flag must be set to false in webpack.config.js.

Note that Angular builds do this automatically, so no change is necessary.