pipewire: Destroy registry object with remote

Currently, the PipeWire registry object is destroyed when
discover_node_factory_sync finishes its functionality. This causes
registry events to not be delivered to the global_removed_cb and
global_added_cb callbacks after discover_node_factory_sync, breaking
the functionality of the
org.freedesktop.portal.Camera.IsCameraPresent property in case of
camera device hot(un)plug.

This changes the discover_node_factory_sync function to store the
registry in the PipeWireRemote structure and destroy it in
pipewire_remote_destroy.