Defer session startup until KAMD is up

Otherwise we can session restore occur before kwin has loaded
activities. This means kwin will fail to put windows back on the correct
activity.

Races are avoided because ksmserver will then make a blocking DBus call
to kwin on session restore. DBus guarantees order so it will have also
dispatched the service status to kwin and we know kwin will have handle
that first.

CCBUG: 456873

Testing done:
Added "sleep(10)" into the main function. Everything resumed on the
correct activities.