Last friday (22/04/2016), during an event organized by DotNetToscana, I had a talk about the build of Xamarin apps with Visual Studio Team Services' Hosted Agetn
To make the build works, I had to add two tasks to manage the Xamarin License to the build definitoin: one to activate the license and another one to deactivate it after the build operation.
There is a small (and big at same time) news about these two tasks: they are no more required now.
Indeed, with the most recend update of the VSTS platform, the Hosted Build Agents have already an internal Xamarin license which is automatically activated when they have to perform some build opreations against Xamarin projects.
So, to sum up, if you have or want to create a Xamarin build definition (and use VSTS Hosted Agent) now you don't have to add the license activation and deactivation tasks anymore.
Have a good build :)