No TLS 1.2 support in Xamarin.Forms? Android app cannot connect to TLS 1.2 web sites

Mono now has TLS 1.2 support: http://tirania.org/blog/archive/2016/Sep-30.html
According to this post on stackoverflow it should work on Xamarin:

http://stackoverflow.com/questions/40735964/how-can-i-enable-ntlm-and-tls-1-2-with-xamarin-on-android

Yet in my Xamarin.Forms (Android version) I cannot connect to TLS 1.2 web sites to download an image:

https://randomuser.me/api/portraits/med/men/85.jpg

This is the error I get:

Image Loading: Error getting stream for randomuser.me/api/portraits/med/men/85.jpg: System.Net.WebException: Error: SecureChannelFailure (The authentication or decryption has failed.) ---> System.IO.IOException: The authentication or decryption has failed. ---> System.IO.IOException: The authentication or decryption has failed. ---> Mono.Security.Protocol.Tls.TlsException: The authentication or decryption has failed. –

Why?