use explicit FetchForegroundService

this avoids potential issues with GenericForegroundService
which eg. may block app start.

src/main/java/org/thoughtcrime/securesms/connect/DcEventCenter.java

@@ -11,7 +11,7 @@ import com.b44t.messenger.DcEvent;
 
 import org.thoughtcrime.securesms.ApplicationContext;
 import org.thoughtcrime.securesms.R;
-import org.thoughtcrime.securesms.notifications.FcmReceiveService;
+import org.thoughtcrime.securesms.service.FetchForegroundService;
 import org.thoughtcrime.securesms.util.Util;
 
 import java.util.ArrayList;
@@ -179,7 +179,7 @@ public class DcEventCenter {
         break;
 
       case DcContext.DC_EVENT_ACCOUNTS_BACKGROUND_FETCH_DONE:
-        FcmReceiveService.backgroundFetchDone();
+        FetchForegroundService.stop(context);
         break;
 
       case DcContext.DC_EVENT_IMEX_PROGRESS:

src/main/java/org/thoughtcrime/securesms/notifications/NotificationCenter.java

@@ -168,6 +168,7 @@ public class NotificationCenter {
     public static final int ID_PERMANENT   = 1;
     public static final int ID_MSG_SUMMARY = 2;
     public static final int ID_GENERIC     = 3;
+    public static final int ID_FETCH       = 4;
     public static final int ID_MSG_OFFSET  = 0; // msgId is added - as msgId start at 10, there are no conflicts with lower numbers
 
 

src/main/java/org/thoughtcrime/securesms/service/FetchForegroundService.java

@@ -0,0 +1,61 @@
+package org.thoughtcrime.securesms.service;
+
+import android.app.Notification;
+import android.app.Service;
+import android.content.Context;
+import android.content.Intent;
+import android.os.IBinder;
+
+import androidx.annotation.Nullable;
+import androidx.core.app.NotificationCompat;
+import androidx.core.content.ContextCompat;
+
+import org.thoughtcrime.securesms.R;
+import org.thoughtcrime.securesms.notifications.NotificationCenter;
+
+public final class FetchForegroundService extends Service {
+  private static final Object SERVICE_LOCK = new Object();
+  private static Intent service;
+
+  public static void start(Context context) {
+    GenericForegroundService.createFgNotificationChannel(context);
+    synchronized (SERVICE_LOCK) {
+      if (service == null) {
+        service = new Intent(context, FetchForegroundService.class);
+        ContextCompat.startForegroundService(context, service);
+      }
+    }
+  }
+
+  public static void stop(Context context) {
+    synchronized (SERVICE_LOCK) {
+      if (service != null) {
+        context.stopService(service);
+        service = null;
+      }
+    }
+  }
+
+  @Override
+  public void onCreate() {
+    super.onCreate();
+
+    Notification notification = new NotificationCompat.Builder(this, NotificationCenter.CH_GENERIC)
+      .setContentTitle(getString(R.string.connectivity_updating))
+      .setSmallIcon(R.drawable.notification_permanent)
+      .build();
+
+    startForeground(NotificationCenter.ID_FETCH, notification);
+  }
+
+  @Override
+  public void onDestroy() {
+    stopForeground(true);
+  }
+
+  @Nullable
+  @Override
+  public IBinder onBind(Intent intent) {
+    return null;
+  }
+}

src/main/java/org/thoughtcrime/securesms/service/GenericForegroundService.java

@@ -185,7 +185,7 @@ public final class GenericForegroundService extends Service {
   }
 
   @TargetApi(Build.VERSION_CODES.O)
-  static private void createFgNotificationChannel(Context context) {
+  static public void createFgNotificationChannel(Context context) {
     if(!CHANNEL_CREATED.get() && Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
       CHANNEL_CREATED.set(true);
       NotificationChannel channel = new NotificationChannel(NotificationCenter.CH_GENERIC,