update_engine: remove size_t in FEC write path am: e0a9543b76 am: dfe52f49cc

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3248266

Change-Id: I8306f3dd4d0d448cb8acdaf9baa3458b41eeda5e
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>