Add DesktopFirstListenerManager

This CL adds DesktopFirstListener to let anyone in SysUI and Shell to
observe the state of desktop-first mode.

Flag: com.android.window.flags.enable_desktop_first_listener
Bug: 415120289
Test: DesktopFirstListenerManagerTest
Change-Id: Iddaa56d023db197487ced4c016a2f9be2e6e8ae6