Do not pass Proxy.NoProxy so that the system proxy settings are respe…

…cted (#3060)

Do not pass Proxy.NoProxy to respect system proxy. As documented in https://developer.android.com/reference/java/net/URL.html#openConnection(java.net.Proxy)
Passing `Proxy.NoProxy` causes the connection to ignore all proxies, including the system one. This is not the intended outcome of the proxy code here, so make sure
that whenever `GetJavaProxy` returns `Proxy.NoProxy` we do the right thing and call the parameterless overload of `URL.OpenConnection`.

src/Mono.Android/Xamarin.Android.Net/AndroidClientHandler.cs

@@ -259,10 +259,14 @@ protected virtual IHostnameVerifier GetSSLHostnameVerifier (HttpsURLConnection c
 			while (true) {
 				URL java_url = new URL (EncodeUrl (redirectState.NewUrl));
 				URLConnection java_connection;
-				if (UseProxy)
-					java_connection = java_url.OpenConnection (await GetJavaProxy (redirectState.NewUrl, cancellationToken));
-				else
+				if (UseProxy) {
+					var javaProxy = await GetJavaProxy (redirectState.NewUrl, cancellationToken).ConfigureAwait (continueOnCapturedContext: false);
+					// When you use the parameter Java.Net.Proxy.NoProxy the system proxy is overriden. Leave the parameter out to respect the default settings.
+					java_connection = javaProxy == Java.Net.Proxy.NoProxy ? java_url.OpenConnection () : java_url.OpenConnection (javaProxy);
+				} else {
+					// In this case the consumer of this class has explicitly chosen to not use a proxy, so bypass the default proxy. The default value of UseProxy is true.
 					java_connection = java_url.OpenConnection (Java.Net.Proxy.NoProxy);
+				}
 
 				var httpsConnection = java_connection as HttpsURLConnection;
 				if (httpsConnection != null) {