Track the empty desk correctly inside `applyLoadPlan`

Track the DesktopTaskView through its `deskId` if it is the current
TaskView or the running TaskView. This is done to support emtpy
desk which has no tasks inside.

Bug: 400532675
Test: m
Flag: com.android.window.flags.enable_multiple_desktops_frontend
Flag: com.android.window.flags.enable_multiple_desktops_backend
Change-Id: Id1e7e7efc2a6ac3126d0320d85a3d327eec0f3c1