Cascade based on desk tasks not display tasks

Calculate cascading position based on task in active desk instead of
tasks in current display.

Flag: com.android.window.flags.enable_cascading_windows
Test: atest WMShellUnitTests:DesktopTasksControllerTest
Fixes: 402252345
Change-Id: Idc2dad2c693855d53f1d5d21f9e4e17f85182bfa