authPrompt: Keep password entry on reset

authPrompt: Keep password entry on reset

Usually, logging in or unlocking the session is made asynchronously,
and AuthPrompt properly manages which entry is currently visible.
External code don't rely on any specific entry to be set, since it
is AuthPrompt's responsibility to select the correct one to be shown.

However, there's one specific case where AuthPrompt must preserve
the password entry: on reset. The reset code preserved whatever
entry was currently displayed, but after fe69dacaf1, it always
changes to the username entry.

Make sure to show the password entry on reset.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/999