Implement repository for display focus in SystemUI

This change adds a simple repository that registers a listener for
display focus to WMShell, and provides StateFlow so other SystemUI
components can consume it easily.

Bug: 356109871
Flag: com.android.window.flags.enable_display_focus_in_shell_transitions
Test: adb shell dumpsys activity service com.android.systemui/.SystemUIService FocusedDisplayRepo
Change-Id: Ie9fdd94e470168ee4105661b509e85e5fd334038