Desks: Do not reuse to-be-removed desk

Default desk creation may happen as a result of removing the last desk.
If that desk was no longer associated to any user, it will have been
scheduled for deletion. This means that there can be a race where the
creation request may be received by RootTaskDesksOrganizer before the
last desk is completely deleted, which results in the organizer
providing the DesktopTasksController/Repository a deskId that will soon
have no associated root to it, ultimately causing a crash the next a
task is attempted to be moved into desktop.

This change adds manual tracking of pending root removals, so that those
roots can be ignored on create requests.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 406333979
Test: enter desktop, swipe it up to remove it from overview or through
adb command, then try to re-enter desktop - verify there's no crash

Change-Id: I0699b92217fe54e03e92509124752f0120b3f37a