DialogPresentationController changes the corner radius dynamically during presentation while ensuring that the presented view's corner radius always matches the corner radius of the tracking view which draws the shadow behind the presented view.
* Add DialogPresentationControllerDelegate
* Consider nullability
* Address PR feedback
* Address a little bit more PR feedback
* Address another batch of PR feedback
## Related links
* Bug: #6438
* Component: [Dialogs](https://github.com/material-components/material-components-ios/tree/develop/components/Dialogs)
## Introduction
As we move to theming extensions all components that have a theming extension should use that within it's examples. Currently MDCAlertController has a theming extension and should use it in all examples. Additionally all components within the examples that have a theming extension should use it.
## The problem
Within MDCAlertController examples some components are using the old _Themers_.
## The fix
For all components being used in MDCAlertController examples use a theming extension if available.
## Additional notes
This also cleans up some ivar usage and import statements.
