I've added a .aidl file to my module. Building the module doesn't generate a java file from the interface.
Comment: these files are used to bind to an out-of-process service (e.g. IInAppBillingService.aidl from play_billing sdk), or to a private native service.
To reproduce, just put the attached file in src/com/android/vending/billing and build. a Java file with the same name should be generated and compiled with the module.