Allow non-A/B, non-retrofit devices to override super_partition

Some devices we want to test on, like cuttlefish, may not have a
partition table on any block device Android can see. The partitions are
simply exposed as separate block devices. This means we need to be able
to override the super_partition name to a regular block device name even
on non-A/B devices.

Bug: 113175337
Test: boot test on cuttlefish
Change-Id: I6ff460d0ba7b1e26cb5d60ba446737aa49549c18