Unregister DisplayWindowListener when window closed

To support drag across multi-displays, we added
MultiDisplayVeiledResizeTaskPositioner as an OnDisplaysChangedListener
to DisplayController. However, the listener was never unregistered,
and since a new taskPositioner is created for each window, this
resulted in a memory leak.

This change addresses the issue by:
* Adding a `close()` method to the `TaskDragResizer` interface and its
implementations. For `MultiDisplayVeiledResizeTaskPositioner`, it
removes the listener from `DisplayController`. For other
implementations, it's a no-op.
* Calling `TaskDragResizer.close()` in `WindowDecoration#close()`.

Bug: 399076683
Test: atest; check heap dump to confirm the memory leak is resolved
Flag: com.android.window.flags.enable_connected_displays_window_drag

Change-Id: I14f0c3d04385c06899f846ceaa1962b024cdc62e