Fix regression in SwipeToScene

This CL fixes a regression caused by ag/25992835: now that
MultiPointerDraggableNode.enabled is a lambda (that might never change),
we need to explicitly observe the result of this lambda to reset the
associated pointer input whenever enabled is changed.

Bug: 291071158
Test: atest SwipeToSceneTest
Flag: N/A
Change-Id: I476228f9f5e66d1b69de33b0879f62507ce31980