Fix show-guess-solutionarea in answer-types.js

Test plan:
Verify the radio buttion is updated when scrolling through the timeline in:
http://localhost:8000/exercises/graphing_points_2.html?debug&problem=0&seed=0&activity=%5B%5B%22incorrect-activity%22%2C%22%5B%5B4%2C4%5D%2C%5C%22QI%5C%22%5D%22%2C8%5D%2C%5B%22correct-activity%22%2C%22%5B%5B4%2C-4%5D%2C%5C%22QIV%5C%22%5D%22%2C13%5D%5D

Auditors: alpert, emily