[stats] add metrics for package installer v2

// Successful installation:
$ out/host/linux-x86/bin/statsd_testdrive -s 98141FFBA005QD 263
...
event_metrics {
  data {
    elapsed_timestamp_nanos: 147291968369
    atom {
      package_installer_v2_reported {
        is_incremental: true
        package_name: "com.unity.megacity"
        duration_millis: 1379
        return_code: 1
      }
    }
  }
}
...

// Failed installation:
$ out/host/linux-x86/bin/statsd_testdrive -s 98141FFBA005QD 263
...
event_metrics {
  data {
    elapsed_timestamp_nanos: 94418812060
    atom {
      package_installer_v2_reported {
        is_incremental: true
        package_name: ""
        duration_millis: 37
        return_code: -20
      }
    }
  }
}
...

Test: manual
BUG: 152913040
Change-Id: Ibb02122e7007f8e3afd65cbb98236bc37c0d24db