问如何在xamarin forms android中更改工具栏返回图标

EN

    LoadApplication(new App());

    var upArrow = Resources.GetDrawable(Resource.Drawable.abc_ic_ab_back_mtrl_am_alpha);
    upArrow.SetColorFilter(Resources.GetColor(Resource.Color.white), PorterDuff.Mode.SrcIn);
    ActionBar.SetHomeAsUpIndicator(upArrow);

using AToolbar = Android.Support.V7.Widget.Toolbar;
[assembly: ExportRenderer(typeof(CustomNavigationPage), typeof(NavigationPageRendererDroid))] // APPCOMP
...
public class NavigationPageRendererDroid : Xamarin.Forms.Platform.Android.AppCompat.NavigationPageRenderer // APPCOMP
{
    public AToolbar toolbar;
    public Activity context;

    protected override Task<bool> OnPushAsync(Page view, bool animated)
    {
        var retVal = base.OnPushAsync(view, animated);

        context = (Activity)Xamarin.Forms.Forms.Context;
        toolbar = context.FindViewById<Android.Support.V7.Widget.Toolbar>(Droid.Resource.Id.toolbar);

        if (toolbar != null)
        {
            if (toolbar.NavigationIcon != null)
            {
                toolbar.NavigationIcon = Android.Support.V4.Content.ContextCompat.GetDrawable(context, Resource.Drawable.Back);
                //toolbar.SetNavigationIcon(Resource.Drawable.Back);
            }
        }
        return retVal;
    }
}

public class CustomNavigationPage : NavigationPage
{
    public CustomNavigationPage(Page startupPage) : base(startupPage)
    {
    }
}

public App()
{
    InitializeComponent();

    MainPage = new CustomNavigationPage(new MainPage());
}
...
// In MainPage
private async void Button_Clicked(object sender, EventArgs e)
{
    await Navigation.PushAsync(new TestPage());
}