问Xamarin forms -移除汉堡菜单图标，但保留导航栏

EN

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using Android.App;
using Android.Content;
using Android.OS;
using Android.Runtime;
using Android.Support.V7.Graphics.Drawable;
using Android.Views;
using Android.Widget;
using MesterDetailspage.Droid;
using Xamarin.Forms;
using Xamarin.Forms.Platform.Android.AppCompat;

[assembly: ExportRenderer(typeof(MasterDetailPage), typeof(MyMasterDetailRenderer))]
namespace MesterDetailspage.Droid
{
    public class MyMasterDetailRenderer : MasterDetailPageRenderer
    {

        public MyMasterDetailRenderer(Context context) : base(context)
        {

        }

        protected override void OnLayout(bool changed, int l, int t, int r, int b)
        {
            base.OnLayout(changed, l, t, r, b);
            var toolbar = FindViewById<Android.Support.V7.Widget.Toolbar>(Resource.Id.toolbar);
            if (toolbar != null)
            {
                for (var i = 0; i < toolbar.ChildCount; i++)
                {
                    var imageButton = toolbar.GetChildAt(i) as Android.Widget.ImageButton;
                    var drawerArrow = imageButton?.Drawable as DrawerArrowDrawable;
                    if (drawerArrow == null)
                        continue;
                    bool displayBack = false;
                    var app = Xamarin.Forms.Application.Current;
                    var detailPage = (app.MainPage as MasterDetailPage).Detail;
                    var navPageLevel = detailPage.Navigation.NavigationStack.Count;
                    if (navPageLevel > 1)
                        displayBack = true;
                    if (!displayBack)
                        ChangeIcon(imageButton, Resource.Drawable.test1);
                    if (displayBack) ;
                        //ChangeIcon(imageButton, Resource.Drawable.back1);
                }
            }
        }
        private void ChangeIcon(Android.Widget.ImageButton imageButton, int id)
        {
            if (Android.OS.Build.VERSION.SdkInt >= Android.OS.BuildVersionCodes.Lollipop)
                imageButton.SetImageDrawable(Context.GetDrawable(id));
            imageButton.SetImageResource(id);
        }
    }
}

<MasterDetailPage xmlns="http://xamarin.com/schemas/2014/forms"
            xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
            xmlns:d="http://xamarin.com/schemas/2014/forms/design"
            xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
            mc:Ignorable="d"
            IconImageSource="test1.png"
            xmlns:views="clr-namespace:MesterDetailspage.Views"
            x:Class="MesterDetailspage.Views.MainPage">