Convert int to string for payload sizes am: 1481c0b0fb am: 89e5b6c043 am: 29b48ec21a

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

Change-Id: Ib664fe59a68d1f017212b26351af4c0cb957e88c
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>