[4/N] Desks: Add WCT#removeRootTask()

Removing a root task through WCT#removeTask only removes its leaves but
not itself. A new #removeRootTask API is added to allow removing the
root along with its leaves in one transaction.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 389729324
Test: swipe up from overview to remove desk tile, WM dumpsys shows the
root is gone

Change-Id: I1a7b22040b382091373329298d5bba1755efe850