Generate interfaces for services to implement #9320
Description
Since #1469 we've not had interfaces in our generated code as we need to be able to add service methods without breaking existing code. Abstract classes were the only way to achieve that. Now that we require Java 8 we can have interfaces again as we can use default methods!
I do wonder if the interfaces used here should implement BindableService or whether they should just be the service methods and have some utility function to bind the service.