Cleanup.

src/main/java/de/dennisguse/opentracks/data/models/Altitude.java

@@ -1,5 +1,7 @@
 package de.dennisguse.opentracks.data.models;
 
+import androidx.annotation.NonNull;
+
 import de.dennisguse.opentracks.R;
 
 public abstract class Altitude {
@@ -60,6 +62,7 @@ public abstract class Altitude {
         }
     }
 
+    @NonNull
     @Override
     public String toString() {
         return "Altitude{" +

src/main/java/de/dennisguse/opentracks/data/models/TrackPoint.java

@@ -420,6 +420,7 @@ public class TrackPoint {
         return this;
     }
 
+    @NonNull
     @Override
     public String toString() {
         return "TrackPoint{" +

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

@@ -635,6 +635,7 @@ public class TrackRecordingService extends Service implements TrackPointCreator.
             return Objects.hash(track, latestTrackPoint, sensorDataSet);
         }
 
+        @NonNull
         @Override
         public String toString() {
             return "RecordingData{" +

src/main/java/de/dennisguse/opentracks/services/handlers/TrackPointCreator.java

@@ -112,8 +112,6 @@ public class TrackPointCreator implements BluetoothRemoteSensorManager.SensorDat
 
     /**
      * Got a new TrackPoint from Bluetooth only; contains no GPS location.
-     *
-     * @param sensorDataSet
      */
     @Override
     public void onChange(SensorDataSet sensorDataSet) {