Protect against accidental Apple Pay button clicks

[davidme-stripe]

Summary

When const { paymentIntent, error } = await confirmPlatformPayPayment() is called twice in quick succession (e.g. if the user double taps the button), we'll fail with a "user cancelled" error. This is confusing, so let's instead return a more explicit error.

We'll also adjust the example code so that it tracks whether Apple Pay is in-flight.

Motivation

#1970

Testing

• I tested this manually
• I added automated tests

Documentation

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• I have added relevant documentation for my changes.
• This PR does not result in any developer-facing changes.