In FreeformTaskListener, flag-guard the method `onFocusTaskChanged` so that it is a no-op when

the aconfig flag `ENABLE_WINDOWING_TRANSITION_HANDLERS_OBSERVERS` is
enabled.

Update the `onTaskMovingToFront`method in DesktopTaskChangeListener to
add task to the repository when the task moves to front.

Bug: 332682201
Test: atest FreeformTaskListenerTest
Test: atest DesktopTaskChangeListenerTest
Flag:
com.android.window.flags.enable_windowing_transition_handlers_observers

Change-Id: I194ad62a60a77e7079d9bcd5c5c43ef8b872fa86