improve performance when clearing notifications (#1283)

app/src/main/java/com/keylesspalace/tusky/util/NotificationHelper.java

@@ -65,6 +65,9 @@ import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.concurrent.ExecutionException;
 
+import io.reactivex.Single;
+import io.reactivex.schedulers.Schedulers;
+
 public class NotificationHelper {
 
     private static int notificationId = 0;
@@ -465,13 +468,18 @@ public class NotificationHelper {
 
     public static void clearNotificationsForActiveAccount(@NonNull Context context, @NonNull AccountManager accountManager) {
         AccountEntity account = accountManager.getActiveAccount();
-        if (account != null) {
+        if (account != null && !account.getActiveNotifications().equals("[]")) {
+            Single.fromCallable(() -> {
                 account.setActiveNotifications("[]");
                 accountManager.saveAccount(account);
 
                 NotificationManager notificationManager = (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);
                 //noinspection ConstantConditions
                 notificationManager.cancel((int) account.getId());
+                return true;
+            })
+            .subscribeOn(Schedulers.io())
+            .subscribe();
         }
     }