Refactor desktop specific callbacks...

Outside of FreeformTaskTransitionObserver. This way the desktop observers are
decoupled and and can be called in order.

Test: atest FreeformTaskTransitionObserverTest
Test: atest DesktopInorderTransitionObserverTest
Bug: 421369676
Flag: com.android.window.flags.enable_inorder_transition_callbacks_for_desktop

Change-Id: I8e8527b2bc485fe9e48f344b2a8062227d875511