Avoid unnecessarily add stub task for Desktop task

- ag/27016210 caused a regression for DesktopTaskView, where shouldAddStubTaskView returns false whenever DesktopTaskView have multiple tasks
- This is because for an unknown reason, runningTasks for Desktop task is always a single element array. This will be further investigated in b/249371338
- For now, shouldAddStubTaskView will still match DesktopTaskView if any runningTasks matches any of the tasks

Bug: 333844287
Test: With 2 split tasks, unsplit one, go to Overview, launch the other split, unsplit, go to Overview
Test: Swipe up from desktop
Flag: NONE
Change-Id: I40cff473595a21ad1619bcdbde30bb8bfd600e15