[STL] handling scenarios where scrollable elements disappear

The idea is that the pointerInput controls the gesture only when none of
the descendants cannot consume it (so it always waits in the Main step).

 When the scrollable disappears, it stops consuming the gesture,
allowing the pointerInput to regain control.

Details explained in go/stl-gesture

Test: atest MultiPointerDraggableTest
Test: Manually tested on our demo app and Flexiglass
Bug: 336710596
Flag: NA
Change-Id: I48f48d624ba853a918f49993be277058bc2dfb3a