replace Handler by default one

import android.content.Intent;

import android.content.SharedPreferences;

import android.os.Binder;

import android.os.Bundle;

import android.os.Handler;

import android.os.IBinder;

import android.os.Message;

import android.util.Log;

import androidx.annotation.Nullable;

import com.owncloud.android.lib.common.operations.RemoteOperation;

import com.owncloud.android.lib.common.operations.RemoteOperationResult;

import java.lang.ref.WeakReference;

import java.util.ArrayList;

import java.util.Collection;

import java.util.Map;

    private final int workerAmount = 4;

    private Thread[] threadPool;

    private OwnCloudClient client;

    private OperationHandler handler;

    private Handler handler;

    @Override

    public int onStartCommand(Intent intent, int flags, int startId) {

        startedSync = new ConcurrentHashMap<>();

        threadPool = new Thread[workerAmount];

        client = DavClientProvider.getInstance().getClientInstance(account, getApplicationContext());

        handler = new OperationHandler(this);

        handler = new Handler();

        return START_REDELIVER_INTENT;

    }

        }

    }

    /**

     * Handler for the class

     */

    static class OperationHandler extends Handler {

        private final String TAG = SynchronizationService.OperationHandler.class.getSimpleName();

        private final WeakReference<SynchronizationService> serviceWeakRef;

        OperationHandler(SynchronizationService mOperationService){

            serviceWeakRef = new WeakReference<>(mOperationService);

        }

        @Override

        public void handleMessage(Message msg) {

            Log.i(TAG, "handler.handleMessage()");

            final Bundle data = msg.getData();

            if (data == null || !data.containsKey("thread index") || !data.containsKey("mThreadWorkingState")) {

                return ;

            }

            final SyncWrapper syncWrapper = serviceWeakRef.get().startedSync.get(data.getInt("thread index"));

            if (syncWrapper != null) syncWrapper.setRunning(data.getBoolean("mThreadWorkingState"));

        }

    }

    private void updateFailureCounter(SyncRequest request, boolean success) {

        final SharedPreferences prefs = getSharedPreferences(FAILED_TRANSFER_PREF, Context.MODE_PRIVATE);

        final String failure_key = request.getSyncedFileState().getLocalPath();