Only consider the screen lock shown after the screen window is mapped
This signal is sent back to the ksldapp after the greeter has registered a window with the compositor. Only once ksldapp gets this signal does it release the lock delaying suspend.
With xdgshell/layershell we have a roundtrip of configuring before the window is even exposed, so we move to a different signal. This also helps encapsulate lockscreen code.
Ideally we want to add code tracking frame rendering per screen tracked too, but that would be based off this change.