[controls] fix leak in ImageSource, caused by Task that never completes

[jonathanpeppers]

Context: https://github.com/AdamEssenmacher/MemoryToolkit.Maui/tree/main/samples

Testing the above sample, I saw some odd results in a .gcdump related to UriImageSource inside a CollectionView:

It appears that we await a Task that never completes, which is:

maui/src/Controls/src/Core/ImageElement.cs

Line 129 in 8e4450c

await oldvalue.Cancel();

The Task also holds onto the ImageSource and whatever memory it happened to use. That would be ok if the Task completed.

I could reproduce the problem in a test:

[Fact]
public async Task CancelCompletes()
{
    var imageSource = new StreamImageSource
    {
        Stream = _ => Task.FromResult<Stream>(new MemoryStream())
    };
    await ((IStreamImageSource)imageSource).GetStreamAsync();
    await imageSource.Cancel(); // This should complete!
}

This non-completing Task can occur when changing Image.Source, which would certainly be bad while scrolling a CollectionView!

To fix, I refactored the code slightly and had the problematic case return:

return Task.FromResult(false);