cppreopts: umount_all fstab.postinstall

- Change 'umount /postinstall' to a full umount_all so that
  postinstall-verity device get destroyed.

- No longer set sys.cppreopt=mounted because that incurs even
  more code. Just duplicate the two procedures.

Test: flash, boot, then check `dmctl list devices`, then OTA
Fixes: 129988285
Change-Id: Ic0cdf55b3639632dc5f2659c8bf36b8c3deea92d
Merged-In: Ic0cdf55b3639632dc5f2659c8bf36b8c3deea92d
1 file changed