Add NestedDraggable.shouldStartDrag()

This CL adds an additional callback to NestedDraggable
shouldStartDrag(), which is necessary to be feature complete with
MultiPointerDraggable and that will be used by Communal to make sure
that STL does not consume vertical drags.

Bug: 378470603
Test: atest NestedDraggableTest
Flag: EXEMPT new API not used anywhere yet
Change-Id: Id950af783e7984841bab4f081a97943f4848b79c