Merge onCreateCallbacks when an unassigned create request is found.

When an unassigned create desk request is found in RootTaskDesksOrganizer#createDesk, modify the onCreateCallback such that the original callback executes, then the newly provided one. This prevents a crash where warmed up desks don't get added to the repository.

Bug: 415283499
Bug: 414416393
Test: Manual
Flag: com.android.window.flags.enable_multiple_desktops_backend
Change-Id: I61c635dd7d48ed6947da4d27bd07555ddc667867