Listen to display focus state in Launcher

As part of the Connected Display project, some UI features in
Launcher such as Alt-Tab need to be shown only on the focused
display.

This change adds FocusState, which registers to WMShell, and
provides a listener to other components in Launcher so they can
easily consume the display focus state (following the same pattern
of HomeVisibilityState).

Bug: 356109871
Flag: com.android.window.flags.enable_display_focus_in_shell_transitions
Test: Switching focus between displays and confirm signals are sent to Launcher.
Change-Id: I51d51ac0ce3e24afa363607cdfb41728748a4a5f