Set rollback index location to 1 for system_image

In soong only builds in case the system image is
defined within soong the product properties aren't
used to configure the image description, so we have
to manually set the index to 1 (matching make) to
ensure it doesn't conflict with the vbmeta image that
uses/reserves the index 0 (default).

Change-Id: I41a25baa6bcfbb1ad89295168ec66ce7ef3e61bc