Allow multiple UI packages to be installed

Currently multiple postmarketos-ui-* packages can't be installed at the same time because the 60-autologin.conf file conflicts. If this file is renamed to something else for every UI package they can at least co-exist. This would lead to less problems when trying to switch UI with the package manager and it would make it possible to select the UI on boot if there's a greeter that supports a mobile screen.

For example gdm can be installed on postmarketos and enabled, and then lightdm disabled. gdm works reasonably well on the pinephone (and probably crashes on most other postmarketOS devices) and can select the desktop to boot in to.

An example of GDM running on the pinephone: https://youtu.be/YTtDk6EsH7k?t=34