Always update the thumbnail layer

Fixes header thumbnail in recents animation down because the surface's
layer was 0 (from new sufaceControl) but did not update the layer
because AppWindowAnimator recorded a different layer number. So now
update the layer for thumbnail on each animation step to ensure that it
will be visible.

Test: boot, open an app, press recents
Change-Id: I9398e164ba297352da2f00e22023ad4fc4d3c903
Fixes: 67910612