Convert int to string for payload sizes am: 1481c0b0fb am: 8d20de69d6

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

Change-Id: I3313de8106fddbe91f44729d89f5dcb252998332
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>