Explicitly mark service notifications as ongoing

app/src/main/java/eu/faircode/email/ServiceExternal.java

@@ -137,7 +137,8 @@ public class ServiceExternal extends Service {
                         .setPriority(NotificationCompat.PRIORITY_MIN)
                         .setCategory(NotificationCompat.CATEGORY_SERVICE)
                         .setVisibility(NotificationCompat.VISIBILITY_SECRET)
-                        .setLocalOnly(true);
+                        .setLocalOnly(true)
+                        .setOngoing(true);
 
         return builder;
     }

app/src/main/java/eu/faircode/email/ServiceSend.java

@@ -222,6 +222,7 @@ public class ServiceSend extends ServiceBase implements SharedPreferences.OnShar
                         .setOnlyAlertOnce(!alert)
                         .setDefaults(0) // disable sound on pre Android 8
                         .setLocalOnly(true)
+                        .setOngoing(true)
                         .setPriority(NotificationCompat.PRIORITY_MIN)
                         .setCategory(NotificationCompat.CATEGORY_SERVICE)
                         .setVisibility(NotificationCompat.VISIBILITY_SECRET);

app/src/main/java/eu/faircode/email/ServiceSynchronize.java

@@ -1314,7 +1314,8 @@ public class ServiceSynchronize extends ServiceBase implements SharedPreferences
                         .setDefaults(0) // disable sound on pre Android 8
                         .setCategory(NotificationCompat.CATEGORY_SERVICE)
                         .setVisibility(NotificationCompat.VISIBILITY_SECRET)
-                        .setLocalOnly(true);
+                        .setLocalOnly(true)
+                        .setOngoing(true);
 
         if (lastAccounts > 0)
             builder.setContentTitle(getResources().getQuantityString(