Use background thread instead of main thread for FocusedDisplayRepository

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: Iabff024e47bc94672f174b1e5f1d75038ae2bb5d
1 file changed