Convert int to string for payload sizes am: 1481c0b0fb

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

Change-Id: Ib713073f366bcf7058125ea094183c670339b328
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>