Make nested scrolling tests depend less on implementation

This CL migrates the nested scroll tests in DraggableHandler to higher
level tests that use the Compose testing API. This will allow to replace
the implementation of the nested scroll implementation witout having to
change the tests, and is the last step towards submitting ag/30549444.

Bug: 378470603
Test: atest SwipeToSceneTest
Flag: EXEMPT test refactor
Change-Id: I09a9ffb7fad410cef246b167e6b91560f79ea9fb