Add tests for null views in animateReorder

Adds unit test for the `animateReorder` method in
`StackAnimationController` to ensure it does not crash when the provided
list of bubble views contains null elements.

Original Change: ag/34961761
Bug: 431235865
Test: StackAnimationControllerTest
Flag: TEST_ONLY

Change-Id: I3c591c82d3f9f28bb9b1272b945803e85820551c