New naming for NestedScrollBehavior

The new naming better reflects what happens when you have nested
scrolling.

If you make a scroll gesture that's partially consumed by the scrollable
 and you reach the edge, there are 3 possible behaviors:
- EdgeNoPreview: you don't show the next scene
- EdgeWithPreview: you show a preview of the next scene
- EdgeAlways: you go directly to the next scene.

Test: atest SceneGestureHandlerTest
Bug: 291053278
Flag: NA
Change-Id: I38d24252386f2be338243810978cf781abb87470