mount-android-partitions: prefer partition images over partitions
In case of Volla Phone X for example to facilitate multibooting Android
11 and Ubuntu Touch (via Volla Boot Manager) the setup involves having a
/var/lib/lxc/android/vendor.img
for Halium 10 around as attempting to
use the system /vendor
from Android 11/12 wouldn't work, thus always prefer
potentially existing image files over partitions.