style: Restore password entry width

style: Restore password entry width

Commit removed the fixed width that is used by polkit/keyring
prompts, with the result that the entries now start at their
natural size, and grow as text is entered.

Restore the original behavior, but nest the style under
.prompt-dialog-password-layout (polkit/keyring) to better
differentiate it from entries under ..prompt-dialog-password-grid
(mount operations).

Fixes: 8d065b558e ("style: Clean up dialogs stylesheet to fix layout issues")
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3511>