largest-series-product: make classes and methods package private #1052 (#1067)

* make classes and methods package private #1052

* method accessor changes

exercises/largest-series-product/.meta/src/reference/java/LargestSeriesProductCalculator.java

-public final class LargestSeriesProductCalculator {
+final class LargestSeriesProductCalculator {
 
     private final String stringToSearch;
 
-    public LargestSeriesProductCalculator(final String stringToSearch) throws IllegalArgumentException {
+    LargestSeriesProductCalculator(final String stringToSearch) throws IllegalArgumentException {
         this.stringToSearch = stringToSearch;
         validateStringToSearch();
     }
 
-    public long calculateLargestProductForSeriesLength(final int seriesLength) throws IllegalArgumentException {
+    long calculateLargestProductForSeriesLength(final int seriesLength) throws IllegalArgumentException {
         if (seriesLength < 0) {
             throw new IllegalArgumentException("Series length must be non-negative.");
         } else if (seriesLength == 0) {