Interval statistics: show unit in dropdown.

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

@@ -75,5 +75,10 @@ public class IntervalStatisticsModel extends AndroidViewModel {
         public int getValue() {
             return value;
         }
+
+        @Override
+        public String toString() {
+            return "" + value;
+        }
     }
 }