Don't expect another task to become focused when dismissing Desktop task

- Don't consider Dekstop task as focused in TAPL
- Renamed variables in dismissTasks_whenDesktopTask_IsInTheCenter as there isn't always a focused task

Fix: 391354508
Test: TaplTestsOverviewDesktop#dismissTasks_whenDesktopTask_IsInTheCenter
Flag: EXEMPT BUG_FIX
Change-Id: Iadc0cf81f37d537837e565e233d11c4148cf3045