[AndroidClientHandler] Prevent orphaned thread in DoProcessRequest

[leonluc-dev]

Followup to #321

In the current implementation the WaitOne task will never finish if the cancellationToken is never cancelled.
This results in an orphaned task/thread.

This can be solved by creating a CancellationTokenSource for the WaitOne thread and linking it to the original cancellation token. This CancellationTokenSource can then be canceled after the task completes or raises and exception, making sure the WaitOne task runs to completion and is properly disposed of every run of this method.

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[jonpryor]

@grendello: Can you review?

@gameleon-dev: Would it be possible to create a unit test for this? It doesn't seem like it would require a dedicated server endpoint, so it seems like it should be possible to unit test this...

Unit tests would go into src/Mono.Android/Test/Xamarin.Android.Net/AndroidClientHandlerTests.cs.

[leonluc-dev]

While it seems difficult to unit test the issue this pull request addresses (orphaned thread), it should be possible to test if the cancellation requests are handled properly with something like the following psuedo code:

    [Test]
    public void Cancel_Client_Works()
    {
      var cts = new CancellationTokenSource();
      cts.Cancel(); //Cancel immediately
      using (var c = new HttpClient(CreateHandler())) {
        Task tr = c.GetAsync("http://10.255.255.1", cts.Token);
        Assert.Throws<System.OperationCanceledException>(tr.Wait(), "SHOULD NOT HAPPEN: Handler did not throw cancellation exception");
       }
    }

    [Test]
    public void Timeout_Client_Works()
    {
      var cts = new CancellationTokenSource(5000); //Cancel after 5000ms
      using (var c = new HttpClient(CreateHandler())) {
        Task tr = c.GetAsync("http://10.255.255.1", cts.Token);
        Assert.Throws<System.OperationCanceledException>(tr.Wait(), "SHOULD NOT HAPPEN: Handler did not throw cancellation exception");
        Assert.IsTrue(cts.IsCancellationRequested, "SHOULD NOT HAPPEN: The request cancelled before cancellation was requested");
      }
    }

I will try to write a unit test based on the above psuedo code and add it to this pull request.