main/postmarketos-ui-plasma-mobile: add support for libhybris backends

kwin can autodetect what backend to use. However, when using the framebuffer we need to set some extra variables.

This MR makes the launch script for Plasma check if either DRM is used (by checking if /dev/dri exists as a directory), or libhybris (by checking if /system exists as a directory, can someone confirm that directory exists on libhybris devices?). If one of those backends is used, the script will let kwin autodetect it. If not, it'll use the framebuffer backend like before.

I can't add him as approver, but I'd like at least @Danct12 to confirm this works on a libhybris device. I tested it myself for DRM and framebuffer using Qemu.