main/postmarketos-ui-phosh: add support for automounting disks

This installs/starts udiskie, a udisk2 helper, that facilitates auto-mounting of removable drives.

Note support for umounting is broken in the current Portfolio release, so disks have to be umounted manually:

udiskie-umount /run/media/<user>/*

I decided to add this to the depends and not pmb_recommends, because I felt like 1) this is a feature that folks would expect to work, 2) by having it in depends, it'll 'fix' existing installs where mounting removable disks automatically is not implemented