Add _b to super.

During OTA, update_engine expects both slots in super,
so that it doesn't have to create the missing slot. Thus,
UUIDs of all logical partitions are consistent. Values
are the result of `uuidgen.py {name}_{slot}` for A/B and
`uuidgen.py {name}` for non-A/B).

Test: adb shell lpdump /dev/block/by-name/super
Bug: 110717529
Change-Id: Ibb90d4dbe1a56bf314d4a02d7aa2f2be18d04855