Activate activity manager asynchronously
Synchronized dbus activation of the activity manager causes a deadlock in kwin_wayland.
Since kactivitymanagerd uses QApplication, it makes a roundtrip to the compositor at startup. However, since kwin's main thread is blocked waiting for the manager to get started, no events are dispatched.
This change makes kactivities start the activity manager asynchronously to prevent deadlocking kwin.
Since Plasma 5.12 is a very old release that is no longer supported by KDE, the fallback code path was dropped.