Register objectPath before service

Summary:
When the service is regsitered we want to have all the internal routing
pre-configured. Otherwise pending calls (such as pending DBus activated
calls) can be processed before the path is registered and then fail with
an error that the path does not exist.

Also remove the fairly redundant check on registerObject. That can return
false if some object in this process has registered itself on that path; but in that
case, the very last thing we'd want to do is call unregisterService.

Test Plan:
Generated a load of notifications
Started plasma
Ouptut showed valid return on GetCapabilities, not an error on missing path

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D7596