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.

CCBUG: 316734