I am developing a payment processing app for Android, and I want to prevent a hacker from accessing any resources, assets or source code from the APK file.<
As someone who worked extensively on payment platforms, including one mobile payments application (MyCheck), I would say that you need to delegate this behaviour to the server, no user name or password for the payment processor (whichever it is) should be stored or hardcoded in the mobile application, that's the last thing you want, because the source can be understood even when if you obfuscate the code.
Also, you shouldn't store credit cards or payment tokens on the application, everything should be, again, delegated to a service you built, it will also allow you later on, be PCI-compliant more easily, and the Credit Card companies won't breath down your neck (like they did for us).