Add test for STL contents zIndex

This CL adds a test for the zIndex logic in STLImpl. This would have
caught the regression at b/435361598, caused by ag/34879256.

Bug: 435361598
Test: atest SceneTransitionLayoutTest
Flag: TEST_ONLY
Change-Id: Ib9b702f27efd37012aa063b3828e51760e846f7d