AnnouncementPeriodicTask: acquire sharedprefences on start (so it can be restarted).

Introduced in 15fbd90ca4.
Fixes #665.

src/main/java/de/dennisguse/opentracks/services/tasks/AnnouncementPeriodicTask.java

@@ -117,13 +117,14 @@ public class AnnouncementPeriodicTask implements PeriodicTask {
         audioManager = (AudioManager) context.getSystemService(Context.AUDIO_SERVICE);
         contentProviderUtils = new ContentProviderUtils(context);
 
-        sharedPreferences = PreferencesUtils.getSharedPreferences(context);
     }
 
     @Override
     public void start() {
         Log.d(TAG, "Start");
 
+        sharedPreferences = PreferencesUtils.getSharedPreferences(context);
+
         if (tts == null) {
             tts = new TextToSpeech(context, status -> {
                 Log.i(TAG, "TextToSpeech initialized with status " + status);