Change recommended pattern

[dotMorten]

In the readme file, it's stated you should use the library this way:

var httpClient = new HttpClient(new NativeMessageHandler());

However on Android this is bad practice. Instead the practice should be to "just" create an HttpClient with the empty constructor. An app can then choose the handler they want to use app-wide for the entire app and all 3rd party libraries using the environment settings, with this single line call at start-up:

 System.Environment.SetEnvironmentVariable("XA_HTTP_CLIENT_HANDLER_TYPE", "ModernHttpClient.NativeMessageHandler,ModernHttpClient");

The problem with explicitly declaring a message handler, is that it completely overrides the http handler selection and forces whichever one you decided at that spot. So libraries shouldn't be doing this, as it prevents app developers from using whichever handler they want app-wide.

You can see how all this works here:
https://github.com/xamarin/xamarin-android/blob/0c26b71339a1027902f72e3842077feb7d9c7d9b/src/Mono.Android/Android.Runtime/AndroidEnvironment.cs#L319-L328

It is also documented here:
https://developer.xamarin.com/guides/android/advanced_topics/environment/
https://developer.xamarin.com/guides/android/application_fundamentals/http-stack/