DO NOT MERGE ANYWHERE Use protobuf for trunk_staging This change causes trunk_staging to use the protobuf implementation of build flagging, and makes it fail if using legacy build flag processing. Bug: 328495189 Test: manual, TH Change-Id: I663a23bea7fc0af6bfb3e6e0a80c7636cfc3a2ad Merged-In: I663a23bea7fc0af6bfb3e6e0a80c7636cfc3a2ad