Remove root permission requirement

The original solution to access the content in an image file is
'mount' which needs root permision. Replaces 'mount' with 'debugfs',
which does not need root permission and will be useful on server
side usage.

Bug: 72027809
Test: ./gsi_util.py pull --system system.img /system/build.prop
Change-Id: I7d6d7b75527cd144d55465353531597afa3baedb