Create onTaskMovedToFront callback.

Create a new onTaskMovedToFront callback that launcher can use to register with
shell. This new callback is based on shell transitions and uses
TaskStackTransitionObserver to update launcher.

Ignore-AOSP-First: new file so it shouldn't conflict
Flag: com.android.window.flags.enable_task_stack_observer_in_shell
Test: atest TaskStackTransitionObserverTest
Test: atest RecentTasksControllerTest
Bug: 341932484
Bug: 344684650
Change-Id: I0d50637cfd06ecf8a41966dbc410266eaa21954a