Support mounting the odm image

Some checkers could read information from odm image. e.g.
sepolicy_checker could also check the compatibility between
system image and odm image. The patch revises the common image source
argument parser and CompositeMounter to support the odm image.

Bug: 72720671
Test: gsi_util pull --odm $OUT/system.img /odm/system/build.prop
Change-Id: I586decaf96640679c7a8e7ec52086d55510fbdeb