When there are 2 markers, the center was translated/shifted too much.
We assume that the width value of markers is `SHAPE_ACTIVITY_MARKER_ICON_SIZE`.
We want to have a spacing value between the 2 markers of `SHAPE_ACTIVITY_MARKER_ICON_MARGIN`.

To have 2 markers horizontally centered and closed to each other, we need to translate the first marker to the left and
the second to the right by half the value of the marker width.
To add a space between the 2 markers, we need to translate both markers by half the value of the global "spacing value".
Previously, the translation value included the full marker width which is too much.

Also include refactoring:
  - Use more accurate name for the parameters of `getMarkerIconOriginFunction`.