prompt: Only show when we got a request

So far we showed the prompt right away which led to some flicker when the UI elements were made visible and can also lead to an empty dialog when non of system_prompt_{password,confirm}_async are ever invoked.

Seems there is a behavior change and the secret code might now call for a dialog it never uses.

Closes: #1092 (closed) Signed-off-by: Guido Günther agx@sigxcpu.org