Make PanModalPresentationController's methods open so they can be overriden by a subclass

[bartosz-treeline]

Summary

Let's say I want to create a presentation controller that has a similar behaviour as PanModalPresentationController. I want to be able to create a PanModalPresentationController's subclass and make some adjustments.
For example I want to create a DrawerPresentationController that inherits from PanModalPresentationController and override containerViewWillLayoutSubviews in order to be able to add more views to the container (e.g. close button that closes a drawer once tapped).

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.

• I've read and agree to the Code of Conduct.

• I've written tests to cover the new code and functionality included in this PR.

[CLAassistant]

All committers have signed the CLA.