Ensure AVB rollback_index_location >= 1

Index 0 is reserved for the vbmeta partition.

This works even for chained vbmeta partitions
as the "father" index location is used instead
of the one bundled within the image description.

Test: aosp_arm64; m --soong-only
Change-Id: Ibb033878e4e56e77b1621a32ba1f2ba7d8ceeedd