Adding /odm support

Treble system-as-root GSI needs to work on devices with and without /odm
partition. This change creates the following symlinks for devices without
/odm partition:

  /odm/app ->/vendor/odm/app
  /odm/bin ->/vendor/odm/bin
  /odm/etc ->/vendor/odm/etc
  /odm/firmware ->/vendor/odm/firmware
  /odm/framework ->/vendor/odm/framework
  /odm/lib -> /vendor/odm/lib
  /odm/lib64 -> /vendor/odm/lib64
  /odm/overlay -> /vendor/odm/overlay
  /odm/priv-app -> /vendor/odm/priv-app

For devices with odm partition, /odm will be used as the mount point and
those symlinks will be hidden after mount.

Bug: 70678783
Test: boot a A/B device without /odm partition, checks those symlinks exist
Test: boot a A/B device with /odm partition, checks those symlinks don't exist
Change-Id: I4d960b52e2754f461188f3ab562c751f502046ea