build_image: Query the actual image size after invoking the final mkfs.

It's not guaranteed that the requested image size to mkfs is precisely
respected, due to metadata alignment and such. For accurate care maps
use the real image size rather than requested.

Bug: 205541521
Test: smartsync to 7892270, check that care_map.pb has the right block
      count
Change-Id: I60fe64f720db13d3c3c4f1d8968341d7293217c9