Fix ServerCertificateValidationCallback after redirect

[simonrozsival]

Fixes #7650

We check if the server certificate hostname matches the request's RequestUri. When the request is redirected to a different host, the verification failed because we tested the next server's hostname against the original request URI. This PR keeps the RequestUri in sync with the redirect status to avoid this problem.

[jonpryor]

@simonrozsival: why is this 2? Are there any docs around how many times ServerCertificateCustomValidationCallback will be invoked, other than "at least once"? Why is the callback being called twice? Is that a bug? Is that likely to change at any point in the future?

Or should it be 2: once for httpbin.org/redirect-to, and once again for www.microsoft.com, and it's called "once per redirect", i.e. if you "nested" redirects with https://httpbin.org/redirect-to?url=https://httpbin.org/redirect-to?url=…, callbackCounter should be the number of times we hit httpbin.org/redirect-to + 1?

[simonrozsival]

The callback is called once for every sub-request that's made (the first request + all redirects). In this case, it's called once for httpbin.org/... and once for www.microsoft.com. So callbackCounter should equal to 1 + number of redirects. The SocketsHttpHandler behaves the same way.

[jonpryor]

Draft commit message:

[Mono.Android] ServerCertificateValidationCallback() and redirects (#7662)

Fixes: https://github.com/xamarin/xamarin-android/issues/7650

`AndroidMessageHandler` was incorrectly validating SSL certificate
after a redirect to another domain if and only if
`AndroidMessageHandler.ServerCertificateCustomValidationCallback`
was set; consider:

	const string url = "https://httpbin.org/redirect-to?url=https://www.microsoft.com/";
	using var handler = new AndroidMessageHandler() {
	    ServerCertificateCustomValidationCallback = (message, certificate, chain, errors) => errors == SslPolicyErrors.None
	};
	using var client = new HttpClient(handler);
	await client.GetAsync(url);

The expectation is that
`AndroidMessageHandler.ServerCertificateCustomValidationCallback`
will be called *twice*:

 1. Once with `message.RequestUri` set to
    `https://httpbin.org/redirect-to?…`, and
 2. Once again with `message.RequestUri` set to
    `https://www.microsoft.com`.

The problem was that in actuality the 2nd invocation was unchanged
from (1), with `message.RequestUri` set to
`https://httpbin.org/redirect-to…?…`, and `errors` set to
`SslPolicyErrors.RemoteCertificateNameMismatch` (!).

The problem was that `AndroidMessageHandler.DoSendAsync()` didn't
update `HttpRequestMessage.RequestUri` to the redirected URI, causing
the `errors` enum value to contain an error, and the second
`ServerCertificateCustomValidationCallback` invocation to have the
wrong information.

Update `AndroidMessageHandler.DoSendAsync()` so that when dealing
with an HTTP redirect, `message.RequestUri` always contains the
"current" value in the redirect chain.  This keeps `RequestUri` in
sync with the redirect status to avoid this problem.