Interval stats: while user is recording a track intervals are refreshed every 5 seconds.

Interval stats: completed the list of split intervals: 1, 2, 3, 4, 5, 10, 20 or 50 km or mi.
Fixes #87.

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

@@ -72,7 +72,9 @@ public class IntervalStatisticsModel extends AndroidViewModel {
         OPTION_3(3),
         OPTION_4(4),
         OPTION_5(5),
-        OPTION_10(10);
+        OPTION_10(10),
+        OPTION_20(20),
+        OPTION_50(50);
 
         private int value;