screenshot: Unrestrict PickColor

screenshot: Unrestrict PickColor

Commit dd2cd6286cd3 restricted callers of the screenshot methods to
portal implementations, gnome-settings-daemon and gnome-screenshot.

That restriction does make sense for the actual screenshot methods,
but `PickColor` is actually used by GTK in its color picker (and
therefore may be called from arbitrary applications).

Fix this by unrestricting access to `PickColor` again. Considering that
the method is always interactive, it's not very privacy/security-sensitive
anyway.

https://gitlab.gnome.org/GNOME/gtk/-/issues/4283

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1990>