Move PaymentSheet example into its own app.

[brnunes-stripe]

Summary

Move existing PaymentSheet example into its own app.

Reads backend address and publishable key from gradle.properties, similarly to existing example app:

# Set to PaymentSheet example backend that implements the '/checkout' endpoint
STRIPE_PAYMENTSHEET_EXAMPLE_BACKEND_URL=https://stripe-mobile-payment-sheet-test-playground-v3.glitch.me/

# Set to a test publishable key from https://dashboard.stripe.com/test/apikeys
STRIPE_PAYMENTSHEET_EXAMPLE_PUBLISHABLE_KEY=pk_test_testkey

Instructions will be added to README later.

Motivation

Making PaymentSheet demo integration simpler.