[20/N] Desks: Implement IDesktopTaskListener add/remove/activation APIs

Forwards desk-level changes seen by the DesktopRepository to listeners
registered through IDesktopMode.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 393961813
Test: atest WMShellUnitTests
Change-Id: Id551692e0b3dd35fe97ae7a35e32d3fbe1fe4eb7