Changed new_cow_size var type to auto to prevent overflow. am: d4806f80d8

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2364279

Change-Id: I78c816b102287ff8a1ad4f7b1859ba75c62e09dc
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>