Correctly dispose of input channels.

Previously, DragResizeInputListener did not dispose of input channels
when they were registered on a background thread and then
DragResizeInputListener was closed before the channels were populated on
the main thread. This change adds the necessary cleanup.

Flag: com.android.window.flags.enable_drag_resize_set_up_in_bg_thread
Bug: 401999837
Test: DragResizeInputListenerTest
Change-Id: I0c6f1e5e9de5df9ea5918fb9926e70e444842f77