secret-handshake: update starter implementation

exercises/secret-handshake/src/main/java/HandshakeCalculator.java

-final class HandshakeCalculator {
+import java.util.List;
 
+final class HandshakeCalculator {
 
+    List<Signal> calculateHandshake(int number) {
+        throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
+    }
 
 }