Ensure docked divider is below always-on-top stacks.

We need interleave it with the AppWindowToken hierarchy even
though it only has a WindowToken.

Fixes: 70318142
Fixes: 70675228
Fixes: 70178829
Test: Manual. ZOrderingTests.
Change-Id: I7b898f2157de85433416579efae5e01a9dcbf46c