错误信息：Error: Exception while loading assemblies: System.IO.FileNotFoundException:
Could not load assembly ‘Mono.Android.Support.v4, Version=0.0.0.0, Culture=neutral, PublicKeyToken=84e04ff9cfb79065’.
Perhaps it doesn’t exist in the Mono for Android profile
从Xamarin.Android 5.1开始，已经没有mono.Android.Support.v4了，使用Xamarin.Android.Support.v4替代。所以，需要使用NuGet安装该组件。