Avoid race on startup dispatching DBus methods

Since mid-Qt5 DBus is processed on another thread, this will return an
error directly if a path is not registered at the time of handling a
request.

This introduces a lot of new races. We need to have our spy hook
registered early. This is safe to do before KDED::self is created as the
implementation will post an event to the main thread.

CCBUG: 425315