xwayland: Use the right atom type for further selection requests to X11

If we translate between text/plain;charset-utf-8 from the wayland side to
UTF8_STRING on the X11 side, we want to continue all further X11 selection
requests using the same translated UTF8_STRING atom than we use in the
first XConvertSelection call.

https://bugzilla.gnome.org/show_bug.cgi?id=782472