fix: issue #3. add private constructor for Mapping class.

src/main/java/de/dennisguse/opentracks/viewmodels/Mapping.java

@@ -10,6 +10,11 @@
 
 public class Mapping {
 
+    // Private constructor to hide the implicit public one
+    private Mapping() {
+        throw new IllegalStateException("Utility class");
+    }
+
     public static Map<String, Callable<StatisticViewHolder<?>>> create(Context context) {
         HashMap<String, Callable<StatisticViewHolder<?>>> m = new HashMap<>();
         m.put(context.getString(R.string.stats_custom_layout_total_time_key), GenericStatisticsViewHolder.TotalTime::new);