Fix scroll and velocity dispatching in NestedDraggable

The nested scroll and fling logic introduced in ag/30548674 was quite
wrong. This CL fixes it and adds tests to make sure that the available
scrolls and velocities we get match what we expect.

Bug: 378470603
Test: atest NestedDraggableTest
Flag: EXEMPT NestedDraggable is not used yet
Change-Id: Ib8d59768e2529ea9863ed26afea7fa67c705dce9