Persist Locus Id Event to UsageStatsProto.

Currently, we are only using UsageStatsProto for backup and restore
purposes but to ensure it is in sync with UsageStatsProtoV2, adding the
recently added LOCUS_ID event. This enables us to easily fall back to
the earlier proto version if there are issues with V2. This also allows
us to backup all events data if we decide to do that in the future
before updating the B&R restore logic to use V2.

Bug: 155935953
Test: atest UsageStatsDatabaseTest
Change-Id: I792f20a0b44d13f1ed17c4dbebe5555044afa165