Add an API to desktop mode to stash desktop apps

Keep track of the state that apps are stashed.
When apps are stashed, always launch new apps on that display on
desktop. Also bring other desktop tasks to front at the same time.

Bug: 261234402
Test: atest DesktopTasksControllerTest DesktopModeTaskRepositoryTest
Change-Id: Ia2ee633a779d0cb438490e90fc84f1cfda10e1fc