GPX import/export: speed via TrackPointExtension/v2.

Fixed data type for heartrate/speed (no decimals) allowed.
Fixes #335.
Fixes #181.

GPXTrackWriter: set Trackpointv2 header correctly and export time in correct sequence.

src/androidTest/java/de/dennisguse/opentracks/content/SearchEngineTest.java

@@ -58,7 +58,6 @@ public class SearchEngineTest {
 
     private final Context context = ApplicationProvider.getApplicationContext();
 
-
     @Before
     public void setUp() {
         providerUtils = new ContentProviderUtils(context);