Fix filtering of DesktopTaskViews

When finding last active tasks we didn't account for the fact that there
could be more than 2 tasks in a GroupTask. This cl fixes that so it
finds the correct last active tasks in desktop cases.

Also make sure that if a  DesktopTaskView is found, we don't launch it
through TaskView#LaunchTasks to create a transition that we can handle
in WM Shell.

Fix: 327447672
Fix: 354171747
Flag: EXEMPT Bugfix
Test: atest SplitSelectStateControllerTest
Change-Id: I71cc630a79f23e5eaad10b3c2284496422ce6994