simulate_ota.py: truncate partitions to multiples of 4 KiB am: a2e0fe38b4

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2083404

Change-Id: I190e8a0200cdca1994c6ebd042dda76e56eb49f9
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>