[iOS] Do not use the background check in the iOS handler.

[mandel-macaque]

The iOS NSUrlSessionHandler needed to use a workaround to ensure that
Http requests that were performed by the application did not end up in a
stale situation in which the completion would never be reached. This was
due to xamarin/xamarin-macios#5463

This workound will listen to the UIApplication notifications and would
cancel all the inflight requests. For reference, this happens here:

https://github.com/xamarin/xamarin-macios/blob/main/src/Foundation/NSUrlSessionHandler.cs#L174

As you can see, if we set the property to false, we will get the
notification. This is not longer needed as mono fixed the runtime
issue. The Xamairn.iOS team skips that workaround by default as per
this commit:

xamarin/xamarin-macios@b6fbae5#diff-1e7e2b8b4f4fe98a485a36c085a0e9cb94f53fa87a3ff11c7c54682cc3b9d514

This library should not be using the workaround because it will have
undesired side effects on applications because they will have
cancelation exceptions that are not expected AND the library does not
provide an API to reuse the HttpClient instance configured by the
client.

[jennyf19]

#2556

[parkhk10]

We have confirmed that the issue originates from the HTTP client handler instance created by MSAL as the stack trace points to NSUrlSessionHandler even when we set our app's client to use a managed handler instead. Any changes to the client handler we make in our app code are not propagated to MSAL, which cancels our requests when the app is backgrounded hence the cancellation exception in the stack trace.