On iPad, with a presentation style of .formSheet, the LockViewController
could be dismissed by simply tapping outside the bounds of the modal,
without regard for the `closable` [1] option.

Setting `isModalInPresentation` [2] based on `closable` allows controlling
whether or not UIKit should prevent the interactive dismissal of the popup.

[1] https://auth0.com/docs/libraries/lock-swift/lock-swift-configuration-options#closable
[2] https://developer.apple.com/documentation/uikit/uiviewcontroller/3229894-ismodalinpresentation

Call onCancel when the Lock view controller modal is dismissed by the user
(by tapping outside its bounds on iPad).

This is not called when it's dismissed programmatically:
https://developer.apple.com/documentation/uikit/uiadaptivepresentationcontrollerdelegate/3229889-presentationcontrollerdiddismiss