Add preliminary HTTPS handling support for Android.

The problem is that Android only exposes Java APIs for ~everything of
consequence, including/especially X509 Certificate validation.  Since
we want Mono-based HTTPS support to use the underlying platform
certificates, this requires some amount of Java-using glue.  However,
we don't want to pull in a ton of Java glue into System.dll (bleh!),
so we'll instead place the glue into Mono.Android.dll's
Android.Runtime.AndroidEnvironment.TrustEvaluateSsl(), and instead
create a delegate which will invoke TrustEvaluateSsl() when needed.

mcs/class/System/System.Net/ServicePointManager.cs

@@ -402,8 +402,19 @@ namespace System.Net
 			static bool is_macosx = System.IO.File.Exists (MSX.OSX509Certificates.SecurityLibrary);
 			static X509RevocationMode revocation_mode;
 
+#if MONODROID
+			static readonly Converter<Mono.Security.X509.X509CertificateCollection, bool> monodroidCallback;
+#endif
+
 			static ChainValidationHelper ()
 			{
+#if MONODROID
+				monodroidCallback = (Converter<Mono.Security.X509.X509CertificateCollection, bool>)
+					Delegate.CreateDelegate (typeof(Converter<Mono.Security.X509.X509CertificateCollection, bool>), 
+							Type.GetType ("Android.Runtime.AndroidEnvironment, Mono.Android", true)
+							.GetMethod ("TrustEvaluateSsl", 
+								System.Reflection.BindingFlags.Static | System.Reflection.BindingFlags.NonPublic));
+#endif
 				revocation_mode = X509RevocationMode.NoCheck;
 				try {
 					string str = Environment.GetEnvironmentVariable ("MONO_X509_REVOCATION_MODE");
@@ -498,6 +509,14 @@ namespace System.Net
 				}
 #endif
 
+#if MONODROID
+				result = monodroidCallback (certs);
+				if (result) {
+					status11 = 0;
+					errors = 0;
+				}
+#endif
+
 				if (policy != null && (!(policy is DefaultCertificatePolicy) || cb == null)) {
 					ServicePoint sp = null;
 					HttpWebRequest req = sender as HttpWebRequest;