Convert int to string for payload sizes am: 1481c0b0fb am: 89e5b6c043

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

Change-Id: Ied2a5322e15f4755d6dba61b6a8f38bd565bf5fc
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>