Keeps the task relative z-ordering when moving an existing recent task

In multi-window scenario, multiple tasks are resumed from the top down
and adding the tasks are added to recents respectively. Therefore,
the task order in the recents list is reversed.

Instead of always moving the existing task to top in recents, this CL
finds a proper index and takes the actual task hierarchy into account
to move the task in recents.


Bug: 317488684
Test: manually verified in desktop windowing
Change-Id: Ib4286340ce83810655fefad598cc37968f93a77d