Implement listener for display focus in WMShell

This is the main implementation of the focused display project
on the WMShell side. The main logic is in FocusTransitionObserver,
which has the following functionalities:

- Provides a listener system for display focus to other components
in the same process (e.g. SystemUI) via FocusTransitionListener and
ShellTransitions.
- Provides a listener system for display focus to components
outside of the process (e.g. Launcher) via IFocusTransitionListener
and IShellTransitions.
- Observes all shell transitions, catch display focus changes, and
notify the listeners.

For now, FocusTransitionObserver assumes that display focus change
only happens with one of the following changes in a transition:
- A change with FLAG_MOVED_TO_TOP
- A change that has newly got focus on the display

Bug: 356109871
Flag: com.android.window.flags.enable_display_focus_in_shell_transitions
Test: FocusTransitionObserverTest
Change-Id: If503e24de2420e1298153edcee9eaec2bc4d58ba