libsnapshot: CowWriter - AddCopy API change am: 1528f920b6

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2262186

Change-Id: I05d448abe9baca0293bbdbec0cf5c73d061239e1
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>