Handle gesture started on nested child in SceneTransitionLayout

When you start scrolling one of its children, this mode is activated.
It prevents the rest of the gesture from being used at the end of the
child's scroll
 to pass (accidentally) to the next scene.

Instead, the rest of the gesture will only be used partially to show a
preview of the next scene.

Test: mp SystemUIComposeGallery
Bug: 291025415
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:599ce55847dcd7c8442ee90efc2f8eef2190497d)
Change-Id: Ifaa3b26fbe3d5ef5262a23fa0d3a556d8353321a