Update update_metadata_pb2.py with new fields am: b6437d82f9

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

Change-Id: I0951bc8a4d8952524cd8f1196a7c1b83594dfd5e
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>