Added stubs to LargestSeriesProductCalculator

Make changes per review comments

Made class and methods package private

exercises/largest-series-product/src/main/java/LargestSeriesProductCalculator.java

-public class LargestSeriesProductCalculator {
-
-
+class LargestSeriesProductCalculator {
+    LargestSeriesProductCalculator(String inputNumber) {
+        throw new UnsupportedOperationException ("Delete this statement and write your own implementation.");
+    }
 
+    long calculateLargestProductForSeriesLength(int numberOfDigits) {
+        throw new UnsupportedOperationException ("Delete this statement and write your own implementation.");
+    }
 }