polkitAgent: Only set key focus to password entry after opening dialog

polkitAgent: Only set key focus to password entry after opening dialog

Set the key focus to the password field only after we got a request
(and therefore know that a password is requested) instead of using
`setInitialKeyFocus()`. This way we don't try to focus the password
field by default if we aren't showing it (e.g. in case the user has no
password or is using fingerprint login).

Also we have to move the call to `grab_key_focus()` to happen after
`_ensureOpen()`, because otherwise the ModalDialog will set the focus to
one of the buttons while opening itself.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/788