Use NestedDraggable to handle STL gestures

This CL makes SceneTransitionLayout use Modifier.nestedDraggable(). This
is a pure refactoring that should be invisible to consumers, and should
make it easier to reason about nested scrolling and gesture handling in
STL.

Bug: 378470603
Test: atest PlatformComposeSceneTransitionLayoutTests
Flag: com.android.systemui.scene_container
Change-Id: Ib0af0b57a00401170596c1b4d140068cd6a8dadf