unlockDialog: Create auth prompt on demand

unlockDialog: Create auth prompt on demand

AuthPrompt is the set of actors that contain the user avatar,
the username, and the password entry. With the removal of the
screen shield, the unlock dialog (be it UnlockDialog or the
LoginDialog) is always created, and in the case of UnlockDialog,
so is the auth prompt.

This is problematic, though, since for passwordless accounts,
the simple act of creating AuthPrompt authenticates the user,
and lifts the lock screen.

Create the AuthPrompt on demand in UnlockDialog.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/872