Support overwriting existing signatures when signing OTA am: fc816880bc am: 789fbdd208

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2307447

Change-Id: I458fce59cbae686e462cc114ec648a1be750252d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>