Adding atoms for metrics logging

Define the atom protos for
UpdateEngineUpdateAttemptReported
UpdateEngineSuccessfulUpdateReported

The fields for both atoms are copied from the already reported Tron
counters, except for the source fingerprint. By defining our own proto,
we can have a better insight about each update events. It also makes the
query easier without the server side work to correlate each counters.

Sample output from testdrive shows:
data {
    elapsed_timestamp_nanos: 64932607816092
    atom {
      update_engine_update_attempt_reported {
        attempt_number: 1
        payload_type: FULL
        duration_boottime_in_minutes: 0
        duration_monotonic_in_minutes: 0
        payload_size_mib: 510
        attempt_result: METADATA_VERIFICATION_FAILED
        error_code: DOWNLOAD_METADATA_SIGNATURE_MISMATCH
        fingerprint: "google/walleye/walleye:R/MASTER/eng.xuncha.20190731.151212:userdebug/dev-keys"
    }
  }
}

Bug: 138253582
Bug: 137682371
Test: run statsd_testdrive and check events

Change-Id: Ic502acc8831fe4da0b32a826171d10e9c0f9876d