correct enum_default_value_name on atoms.proto

Test: run runtests.sh, make sure no missing test and get all pass
      run /out/host/linux-x86/bin/statsd_testdrive 103
      Got following metric data dump:pass for local test
Bug: 119862317

Change-Id: I4588cec0dea8a4c5a19847a32a6d53bfc5953be7