Support overwriting existing signatures when signing OTA am: fc816880bc

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

Change-Id: Ife1ed973ce210cebdd3cdd72b97486be9ee017bf
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>