Fix layering issue when launching from side TaskView

Fully reverted the change in ag/24840504, and instead make use of
prefixOrderIndex to ensure translucent overlays launch on top of
the underlying task.

Test: Swipe up from an app to Overview (phone), scroll over one
task, click on the live tile you came from; ensure animates well
Test: Open an app, launch a translucent overlay, e.g. Volume
Dialog, swipe up to Overview and relaunch the live tasks; ensure
the overlay stays on top throughout the animation
Fixes: 310765248
Flag: ACONFIG com.android.launcher3.enable_grid_only_overview TEAMFOOD

Change-Id: I0708bf3c060ba84089722d0bd9480a4f4bd2b8e2