Add estimate_cow_size field to update metadata am: 7d64e28e6d am: 822d3e6f63 am: bd586ea82f

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1419149

Change-Id: Ieb1a63d570bc56b287b92e4559a80b54f1363bb8