Rework stack Z-ordering promotion.

Down-boost home stack instead of up-boosting docked stack. This means
we don't have to do any assistant stack boosting so we wont have issues
with activities not being able to show on top of the assistant.

Change-Id: Iedef49a56a25fe0c6c26bfe4ef5194bcd8159c37
Fixes: 70318142
Fixes: 70675228
Fixes: 70178829
Fixes: 69553456
Fixes: 70178829
Test: Manual. ZOrderingTests. go/wm-smoke