Support custom Xamarin Android HttpClient handler

Make it possible to select a default System.Net.HttpClient handler
on Android. Selection of the handler is handled on the Xamarin Android
side.

mcs/class/System.Net.Http/Makefile

@@ -6,6 +6,9 @@ LIBRARY = System.Net.Http.dll
 
 LIB_REFS = System.Core System
 LIB_MCS_FLAGS = $(EXTRA_LIB_MCS_FLAGS)
+ifeq (monodroid,$(PROFILE))
+LIB_MCS_FLAGS += -d:XAMARIN_MODERN
+endif
 
 TEST_LIB_REFS = System System.Core
 TEST_MCS_FLAGS =

mcs/class/System.Net.Http/System.Net.Http/HttpClient.android.cs

@@ -0,0 +1,38 @@
+using System;
+using System.Reflection;
+
+namespace System.Net.Http {
+	public partial class HttpClient {
+
+		public HttpClient ()
+			: this (GetDefaultHandler (), true)
+		{
+		}
+
+		static HttpMessageHandler GetDefaultHandler ()
+		{
+			Type type = Type.GetType("Android.Runtime.AndroidEnvironment, Mono.Android");
+			if (type == null)
+				return GetFallback ("Invalid Mono.Android assembly? Cannot find Android.Runtime.AndroidEnvironment");
+
+			MethodInfo method = type.GetMethod ("GetHttpMessageHandler", BindingFlags.Static | BindingFlags.NonPublic);
+			if (method == null)
+				return GetFallback ("Your Xamarin.Android version does not support obtaining of the custom HttpClientHandler");
+
+			object ret = method.Invoke (null, null);
+			if (ret == null)
+				return GetFallback ("Xamarin.Android returned no custom HttpClientHandler");
+
+			var handler = ret as HttpMessageHandler;
+			if (handler == null)
+				return GetFallback ($"{ret?.GetType()} is not a valid HttpMessageHandler");
+			return handler;
+		}
+
+		static HttpMessageHandler GetFallback (string message)
+		{
+			Console.WriteLine (message + ". Defaulting to System.Net.Http.HttpClientHandler");
+			return new HttpClientHandler ();
+		}
+	}
+}

mcs/class/System.Net.Http/monodroid_System.Net.Http.dll.sources

@@ -0,0 +1,2 @@
+#include System.Net.Http.dll.sources
+System.Net.Http/HttpClient.android.cs