Fix z order of tasks in overview.

The recent tasks list provided to launcher is ordered from most recent
to least recent. As a result, we need to add snapshot views always to
the start of the view list.

Test: Manual
Bug: 269664273
Bug: 317486696
Flag: None
Change-Id: I4efb3b3dbb21171cbbb52783e88e86d1b1778ec3