Completely rewrite the internals and most of the APIs.

[zach-klippenstein]

• All the actual transition logic is moved out of the Backstack composable and into TransitionController.
• BackstackInspector and InspectionGestureDetector are replaced with a single API in a new, dedicated module: xrayed().
• Backstack now just handles the container and managing saveable state.
• Introduces FrameController, an abstraction which can be used to implement both transitions, the inspector functionality, and things like Consider API for manually controlling transition #17.
• Removes the spring animations from the inspector. They added unnecessary lag to the gestures.
• Adds more tests for state handling.
• Fixed some races that caused navigation animation glitches.

This change fixes a number of issues:

• Closes Don't start composing new screens that are in hidden positions in the backstack until they are first shown #4
• Closes Consider handling low memory callbacks automatically #5
• Closes drawScreen should get an isVisible flag to allow screens to clean up resources they don't need #16
• Closes [DNM] Experimenting with RestorableStateHolder shouldSave API #42
• Closes Use new camera distance API for inspector layer transformations #44 (camera distance API is used to improve the 3D view, but not for zooming itself)
• Unblocks Consider API for manually controlling transition #17