Support custom payload signer in merge_ota am: 6c8f8965

Original change: https://googleplex-android-review.googlesource.com/c/platform/build/+/23289181



Change-Id: I34866da2b67e6810c31377dea4f1c610b4994de0
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>