[metrics] implement callbacks for InstalledIncrementalPackage pulled atom

BUG: 187872906
Test: 1. install 3 incremental apps; 2. run `statsd_testdrive 10114` and
wait to finish; 3. observe that the result contains 3 atoms.

gauge_metrics {
  data {
    bucket_info {
      atom {
        installed_incremental_package {
          uids: 10306
        }
      }
      atom {
        installed_incremental_package {
          uids: 10307
        }
      }
      atom {
        installed_incremental_package {
          uids: 10308
        }
      }
      elapsed_timestamp_nanos: 561691558765
      elapsed_timestamp_nanos: 561691558765
      elapsed_timestamp_nanos: 561691558765
      bucket_num: 9
    }
  }
}
time_base_elapsed_nano_seconds: 14974700031
bucket_size_nano_seconds: 60000000000
is_active: true

Change-Id: I4f1d315818af5096f4d4341de063f6a162ebaf26