URL$DefaultFactory::PREFIX should be static final

https://github.com/openjdk/jdk/blob/724bf3be1458f7da502f8772be6151bed826b4f7/src/java.base/share/classes/java/net/URL.java#L1245

        private static String PREFIX = "sun.net.www.protocol.";

This field is never modified. It should be static final.