64-arphic-uming.conf: Weaken priority and remove modification to DejaVu Serif
After Ubuntu 19.04 and Debian 10, both Ubuntu and Debian have decided to prefer Noto CJK fonts. This adjustment to 64-arphic-uming.conf changes the strong "prepend" relationship of fonts to "append" so that to make sure that arphic fonts do not take priority.
Besides, the modification to the priority of DejaVu Serif are removed as well to solve the issue as originally raised in https://bugs.launchpad.net/ubuntu/+source/fonts-arphic-uming/+bug/1834561 .
Part of downstream Ubuntu's 65-arphic-uming.conf is merged inside to make uming fonts available for serif and monospace (sans-serif excluded).
