Rework HttpWebRequest to not use MOBILE only API

[chamons]

As found here:

• [XM] XM.Full - 'HttpWebRequest' does not contain a constructor that takes 1 arguments xamarin-macios#3953

the HttpWebRequest sample does not compile with Full.

This is due to this API being non-public:

-https://github.com/mono/mono/blob/master/mcs/class/System/System.Net/HttpWebRequest.cs#L153

We should update the sample if possible to use another API.