Bugfix: if sensors are running, do not try to start again.

Fixes #1887.

src/main/java/de/dennisguse/opentracks/services/TrackRecordingService.java

@@ -205,6 +205,7 @@ public class TrackRecordingService extends Service implements TrackPointCreator.
     private synchronized void startSensors() {
         if (isSensorStarted()) {
             Log.i(TAG, "sensors already started; skipping");
+            return;
         }
         Log.i(TAG, "startSensors");
         wakeLock = SystemUtils.acquireWakeLock(this, wakeLock);