[21/N] Desks: Implement IDesktopTaskListener initial sync

Upon registration, updates the listener with the current state of the
desktop.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 393977027
Bug: 393961813
Test: atest WMShellUnitTests
Change-Id: I40f9b26194436d1ee3b8eadfe59eddf47b1d9e50