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

tools/releasetools/build_image.py

@@ -651,6 +651,10 @@ def BuildImage(in_dir, prop_dict, out_file, target_out=None):
   if not mkfs_output:
     mkfs_output = BuildImageMkfs(in_dir, prop_dict, out_file, target_out, fs_config)
 
+  # Update the image (eg filesystem size). This can be different eg if mkfs
+  # rounds the requested size down due to alignment.
+  prop_dict["image_size"] = common.sparse_img.GetImagePartitionSize(out_file)
+
   # Check if there's enough headroom space available for ext4 image.
   if "partition_headroom" in prop_dict and fs_type.startswith("ext4"):
     CheckHeadroom(mkfs_output, prop_dict)