Avoid NPE when nested drag starts before enabling nestedDraggable()

This CL fixes an NPE that was happening when a nested drag was started
right before enabling a nestedDraggable(), leading to a null
lastFirstDown.

Bug: 378470603
Test: atest NestedDraggableTest
Flag: EXEMPT NestedDraggable is not used yet
Change-Id: Ib7a4122b2815b1d1e1a664f04bcc5d6bd76bf2d8