Fail if there are repeated fields in atoms.proto

Repeated fields arent allowed outside of attribution node and key value
pairs, so we should fail to build if someone attempts to add one.

Bug: 149956370
Test: atest stats-log-api-gen-test
Change-Id: I0caa6fcf74dabe8e962360ecccfd3a87fcaa4909
Merged-In: I0caa6fcf74dabe8e962360ecccfd3a87fcaa4909