AnswerArea: Fix input-numbers in the answer area

Summary:
interceptInputFocus was being set on all inputs in the
answer area, making them readonly.

Test Plan:
Answer [this perseus item](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22**Fill%20in%20the%20missing%20value%20to%20make%20the%20table%20represent%20a%20linear%20equation.**%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22%24x%24%20%7C%20%24y%24%5Cn-%3A%7C-%3A%5Cn%241%24%20%7C%20%244%24%5Cn%243%24%20%7C%20%2410%24%5Cn%246%24%20%7C%20%2419%24%5Cn%2412%24%20%7C%20%5B%5B%E2%98%83%20input-number%201%5D%5D%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%22input-number%201%22%3A%7B%22type%22%3A%22input-number%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22value%22%3A%2237%22%2C%22simplify%22%3A%22required%22%2C%22size%22%3A%22small%22%2C%22inexact%22%3Afalse%2C%22maxError%22%3A0.1%2C%22answerType%22%3A%22number%22%7D%7D%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22hints%22%3A%5B%7B%22content%22%3A%22Linear%20equations%20can%20be%20represented%20in%20the%20form%20%24y%3D%5C%5Cblue%7Bm%7Dx%2B%5C%5Cpink%7Bb%7D%24%2C%20where%20%24%5C%5Cblue%7Bm%7D%24%20represents%20the%20slope%20of%20the%20equation%20and%20%24%5C%5Cpink%7Bb%7D%24%20represents%20the%20%24y%24-intercept.%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22Two%20points%20always%20make%20a%20line%2C%20so%20let's%20use%20two%20of%20the%20points%20in%20the%20table%20to%20determine%20the%20linear%20equation%20for%20those%20two%20points.%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22Let's%20find%20the%20slope%20first.%20%20Slope%20is%20the%20change%20in%20%24y%24%20over%20the%20change%20in%20%24x%24.%20%20%5Cn%5CnWe%20can%20find%20slope%20using%20this%20formula%3A%5Cn%24%5C%5Cblue%7B%5C%5Cdfrac%7By_2-y_1%7D%7Bx_2-x_1%7D%7D%24%5Cn%5CnLet%E2%80%99s%20use%20%24%5C%5Cgreen%7B(1%2C4)%7D%24%20and%20%24%5C%5Cpurple%7B(3%2C10)%7D%24%20to%20determine%20the%20slope%3A%5Cn%5Cn%24%5C%5Cblue%7B%5C%5Cdfrac%7By_2-y_1%7D%7Bx_2-x_1%7D%7D%3D%5C%5Cdfrac%7B%5C%5Cpurple%7B10%7D-%5C%5Cgreen4%7D%7B%5C%5Cpurple3-%5C%5Cgreen1%7D%3D%5C%5Cdfrac62%3D%5C%5Cblue3%24%5Cn%5CnSo%20the%20slope%20is%20%24%5C%5Cblue3%24.%20Whenever%20the%20%24x%24-value%20increases%20by%20%241%24%20unit%2C%20the%20%24y%24-value%20increases%20by%20%243%24%20units.%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22Now%20let%E2%80%99s%20determine%20the%20%24y%24-intercept%20by%20plugging%20one%20of%20the%20points%2C%20%24%5C%5Cgreen%7B(1%2C4)%7D%24%2C%20into%20%24y%3D%5C%5Cblue%7Bm%7Dx%2B%5C%5Cpink%7Bb%7D%24.%5Cn%5Cn%24y%3D%5C%5Cblue%7Bm%7Dx%2B%5C%5Cpink%7Bb%7D%24%20%20%5Cn%24%5C%5Cgreen4%3D%5C%5Cblue3(%5C%5Cgreen1)%2B%5C%5Cpink%7Bb%7D%24%20%20%5Cn%244%3D3%2B%5C%5Cpink%7Bb%7D%24%20%20%5Cn%241%3D%5C%5Cpink%7Bb%7D%24%5Cn%5CnSo%20the%20line%20intercepts%20the%20%24y%24-axis%20at%20%24(0%2C%5C%5Cpink%7B1%7D)%24.%20%20%5Cn%24m%3D%5C%5Cblue3%24%20and%20%24b%3D%5C%5Cpink1%24%2C%20so%20the%20linear%20equation%20for%20these%20two%20points%20is%20%24y%3D%5C%5Cblue3x%2B%5C%5Cpink1%24.%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22Now%20that%20we%20have%20the%20linear%20equation%2C%20let%E2%80%99s%20plug%20%24x%3D%5C%5Cred%7B12%7D%24%20into%20the%20equation%20to%20solve%20for%20the%20corresponding%20%24y%24-value%3A%5Cn%5Cn%24y%3D3x%2B1%24%20%20%5Cn%24y%3D3(%5C%5Cred%7B12%7D)%2B1%24%20%20%5Cn%24y%3D36%2B1%24%20%20%5Cn%24y%3D37%24%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22%2437%24%20is%20the%20missing%20number%20that%20will%20make%20the%20table%20represent%20a%20linear%20equation.%20%20The%20table%20should%20look%20like%20this%3A%5Cn%5Cn%24x%24%20%7C%20%24y%24%5Cn-%3A%7C-%3A%5Cn%241%24%20%7C%20%244%24%5Cn%243%24%20%7C%20%2410%24%5Cn%246%24%20%7C%20%2419%24%5Cn%2412%24%20%7C%20%2437%24%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%5D%7D)

Auditors: charlie