Don't hardcode the super partition name.

If a device has logical partitions but does not use a partition called
"super", we will need this to be configurable. Note that the "slot"
argument is currently unused, but will be necessary for update_engine if
there are A and B super partitions.

Bug: 116608795
Test: super partition works
Change-Id: Ieb548a158faeb423f3561230f26d582325efbc6b