Each dimmer can only have one dim layer at a time.

Only allow each Dimmer to control one dim layer at a time.
This is to make sure that there's only one dim per Task, Stack,
or NonAppWindowContainer.

Fixes: 72812390
Test: Open multiple dialogs on top of each other in one app.
Dim will remain the same and not get darker for each dialog.
Test: DimmerTests

Change-Id: I21ebbfb06fa6aefa8a66c5991d86fe22610a8859