Use correct signal name when disconnecting from "NameOwnerChanged"

This is a follow-up to #46.

A disconnectSignal() call with a wrong signal name caused that hook wasn't found and thus kept in QDBusConnectionPrivate::signalHooks forever.

I think it's a good candidate for merging upstream; I'll push the change into Gerrit after your review here.

Assigning a parent to QConnmanServiceInterface instances doesn't help in fixing crash-reporter-autouploader crashes, but I'm proposing this change as well so that the objects are properly destroyed.