Bluetooth: use TrackRecording handler for callback.

Otherwise the thread is not defined and a SecurityException may be triggered when inserting data into the ContentProvider.
Sadly, below API26 this needs to be done manually.

Fixes #1165.

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

@@ -211,7 +211,7 @@ public class TrackRecordingService extends Service implements TrackPointCreator.
 
     private void startSensors() {
         wakeLock = SystemUtils.acquireWakeLock(this, wakeLock);
-        trackPointCreator.start(this);
+        trackPointCreator.start(this, handler);
         showNotification(true);
     }
 

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

@@ -2,6 +2,7 @@ package de.dennisguse.opentracks.services.handlers;
 
 import android.content.Context;
 import android.location.Location;
+import android.os.Handler;
 import android.util.Log;
 import android.util.Pair;
 
@@ -48,12 +49,12 @@ public class TrackPointCreator implements BluetoothRemoteSensorManager.SensorDat
         this.gpsHandler = gpsHandler;
     }
 
-    public synchronized void start(@NonNull Context context) {
+    public synchronized void start(@NonNull Context context, @NonNull Handler handler) {
         this.context = context;
 
         gpsHandler.onStart(context);
 
-        remoteSensorManager = new BluetoothRemoteSensorManager(context, this);
+        remoteSensorManager = new BluetoothRemoteSensorManager(context, handler, this);
         altitudeSumManager = new AltitudeSumManager();
 
         remoteSensorManager.start();