paypal/paypal-android ? reverse-engineered prompt

Build me an Android SDK that lets merchants accept PayPal and card payments in their app, with a clean demo app that shows how to start checkout, handle success and cancel states, and plug in a client ID from the PayPal dashboard.

I want it written in Kotlin, usable from both Kotlin and Java, and compatible with Android 23 and up. Please organize it so someone can add only the payment modules they need, and make sure there’s a simple sample integration they can run right away. If it helps, look up the current PayPal Android docs online so the setup matches the latest guidance.

Also include basic tests, clear README instructions for adding the SDK, and enough documentation that a developer can understand how to use it without digging through the source.