Desks: Deactivate and clean up desk on exit using recents transition

Uses the already registed RecentsMixedHandler to let
DesktopTasksController handle the recents' finish. When the finish is
not returning to the app/desk (e.g. going Home), it appends the
desktop cleanup ops to the finishWCT.

This requires changing DesksTransitionObserver to handle merged
transitions and finished transitions too:
- Merges, because recents transitions finished with a book-end
  transition means the deactivation updates should happen when the
  book-end one is ready
- Finishes, because when there's no book-end transition, by the time the
  deactivation changes are added to the finishWCT and starts getting
  tracked by the observer, the transition has already gone through
  #onTransitionReady, so double-checking in #onTransitionFinished
  ensures the changes are reflected in the DesktopRepository.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 402306937
Bug: 401023534
Test: enter desktop, swipe-to-home - verify new app launches from
taskbar open in fullscreen.
Test: enter desktop, swipe-to-overview, then swipe-to-home - verify new
launches from taskbar open in fullscreen.
Test: enter desktop, swipe-to-overview, select fullscreen tile - verify
new launches from taskbar open in fullscreen.
Test: enter desktop, quickswitch to fullscreen - verify new launches
from taskbar open in fullscreen

Change-Id: I34b9a581db3af5e160e32b005d35742089f55cb7