Respect Z-ordering when assigning fullscreen-dim layer.

As an optimization, we use a shared fullscreen dim layer for multiple
users. We need to be careful to look at the Z-order of various users
when choosing who to assign it to.

Bug: 36415615
Test: Ask assistant question, long press power, assistant should be dimmed.
Change-Id: Ifea94fe33d32236b79800c0e7c543aac52c8b69f