Log various types of back gesture using StatsLog

Bug: 135152789
Test: m statsd_testdrive -j31
Test: out/host/linux-x86/bin/statsd_testdrive 224

<Verified all four types of back gesture>
  data {
    elapsed_timestamp_nanos: 113275964369614
    atom {
      back_gesture_reported_reported {
        type: INCOMPLETE
        y_coordinate: 900
        x_location: LEFT
      }
    }
  }
  data {
    elapsed_timestamp_nanos: 113281680894558
    atom {
      back_gesture_reported_reported {
        type: COMPLETED
        y_coordinate: 912
        x_location: LEFT
      }
    }
  }
  data {
    elapsed_timestamp_nanos: 113285878610809
    atom {
      back_gesture_reported_reported {
        type: INCOMPLETE_EXCLUDED
        y_coordinate: 2496
        x_location: LEFT
      }
    }
  }
  data {
    elapsed_timestamp_nanos: 39490430338
    atom {
      back_gesture_reported_reported {
        type: COMPLETED_REJECTED
        y_coordinate: 856
        x_location: LEFT
      }
    }
  }




Change-Id: I986a64c296cc2a1e33db8f7ea81b05b0ee96cdd5