title: New Bindable Picker can't set default item from viewmodel
body: <p>Trying to get the new bindable picker to work in Xamarin Forms version 2.3.4.270 with Visual Studio 2017.</p>
  
  <p>I can get the view to load in a list from the view model so when you click the picker, it does show the list of items.  I want to set a default item in the picker on screen load though and nothing I have tried is working.</p>
  
  <p>View:</p>
  
  <pre class="lang-xml prettyprint-override"><code>&lt;?xml version="1.0" encoding="utf-8" ?&gt;
  &lt;ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
               xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
               xmlns:local="clr-namespace:BindablePickerTest"
               x:Class="BindablePickerTest.MainPage"&gt;
      &lt;StackLayout&gt;
          &lt;Label Text="Welcome to Xamarin Forms!" 
             VerticalOptions="Center" 
             HorizontalOptions="Center" /&gt;
          &lt;Picker Title="Test Picker" ItemsSource="{Binding Names}" SelectedItem="{Binding Name, Mode=TwoWay}" /&gt;
      &lt;/StackLayout&gt;
  &lt;/ContentPage&gt;
  </code></pre>
  
  <p>View Code Behind: (the view model is in the code behind for simplicity)</p>
  
  <pre class="lang-cs prettyprint-override"><code>using System;
  using System.Collections.Generic;
  using System.ComponentModel;
  using System.Linq;
  using System.Text;
  using System.Threading.Tasks;
  using Xamarin.Forms;
  
  namespace BindablePickerTest
  {
      public partial class MainPage : ContentPage
      {
          MainPageViewModel vm;
  
          public MainPage()
          {
              InitializeComponent();
              this.BindingContext = vm = new MainPageViewModel();
          }
  
      }
  
      public class MainPageViewModel : INotifyPropertyChanged
      {
  
          List&lt;string&gt; names = new List&lt;string&gt;
          {
              "Doug",
              "Fred",
              "Steve",
              "Tom",
              "Victor"
          };
          public List&lt;string&gt; Names =&gt; names;
          public string Name = "Steve";
  
          public event PropertyChangedEventHandler PropertyChanged;
      }
  }
  </code></pre>
  
  <p>When this runs, the picker is still empty - "Steve" is not selected.  When clicking the picker, the list shows correctly and I can choose any name, but it will not set a default selection.</p>
  
  <p>Any help is appreciated!</p>