_InteractiveViewerState code cleanup (#153645)

This pull request makes the interactive viewer's `TransformationController` a non-nullable value, initialized at its declaration site. (related: #153644)

It also renames a few private methods to follow the [style guide](https://github.com/flutter/flutter/blob/master/docs/contributing/Style-guide-for-Flutter-repo.md#naming-rules-for-typedefs-and-function-variables).

Author: nate-thegrate