update_engine: xor writer size_t am: ae222aa0f8

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3343486

Change-Id: I1c78bbb254af27f8331273e37e5469e236ba056d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>