Processing does not integrate properly with the native Android interface mechanism.
Your best bet is to look into APWidgets, but that isn't exactly a full alternative to the native framework.
If you want to work with native Android, then you could try to wrap the Processing sketch into an Android application...

You're right about not being able to integrate the native platform directly... although, it is definitely possible to integrate a Processing Activity into a larger native Android application.

There are a couple other GUI libraries that support Android on the Libraries page, but APWidgets is your best bet... When I've written applications, I've had to write the user interface from scratch.

Guido? The problem is that it's just a framework for developing an interface... it doesn't actually provide components to use. You'd have to write them all yourself...