Use new ScreenControllers class for controller lifecycle management

[kenzieschmoll]

We used to use package:provider to manage the lifecycle of the DevTools screen controllers, and ProvidedControllerMixin as a way to easily initialize and access these controllers. This caused lifecycle management issues and memory leaks because the screen controllers (which are expected to be long living) were tied to the widget lifecycle, since lookup controllers up from package:provider requires access to BuildContext.

This PR creates a class ScreenControllers that is accessible from a global screenControllers. Individual screen controllers are still created lazily upon first access, and can be accessed from anywhere (widget code or business logic) by calling screenControllers.lookup<ControllerType>().

This PR also deletes the ProvidedControllerMixin since it is no longer needed.