AutoService: should override the service config file instead of appending new contents

When generating a service config file (e.g `META-INF/services/serviceloader.Dictionary`), the processor reads the old content from the existing config file, adding new services and then writes to the config file.

The problem I observed is that if I delete a service class (e.g ExtendedDictionary) after the first build and then build and run at the second time, I wll get `java.util.ServiceConfigurationError: serviceloader.Dictionary: Provider serviceloader.ExtendedDictionary not found` error since the old entry `serviceloader.ExtendedDictionary` is still in the newly generated file.

I can use `mvn clean package` to avoid this problem. But if I use some IDE like Intellij, it's not a good practice to have to delete the file manually every time. I don't see why we should append the file instead of overriding.