Update update_metadata_pb2.py with new fields am: b6437d82f9 am: 64b72f3e78

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2204082

Change-Id: I5b33d769ca043634ea06d574dfa7e8e10633eb7f
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>