Add a preupload hook to forbid use of "required" keyword in protobuf am: 6b45ea5c3d

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1567880

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: Icc52fef36fb08c9607d572fde8ae2c2dfac537cf