Convert int to string for payload sizes am: 1481c0b0fb

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2853673

Change-Id: If3a0993d17a21e34de417dc5fbceee4de2e5d5c7
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>