Expression: Fix old expression grading

Summary:
The old expression widget was sending
`{value: this.state.value}` to `validate()` while the new
expression just sent `this.state.value`, causing old
expressions to always grade as wrong.

Test Plan:
In test.html, set useMathQuill to false
Open [this item](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22Write%20an%20expression%20to%20represent%20the%20following%20quantity%3A%5Cn%5Cn%2A%2A%24438%24%20minus%20the%20sum%20of%20%2474%24%20and%20%242%24%2A%2A%5Cn%5Cn%5B%5B%E2%98%83%20expression%201%5D%5D%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%22expression%201%22%3A%7B%22type%22%3A%22expression%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22answerForms%22%3A%5B%7B%22value%22%3A%22438-%5C%5Cleft%2874%2B2%5C%5Cright%29%22%2C%22form%22%3Atrue%2C%22simplify%22%3Afalse%2C%22considered%22%3A%22correct%22%7D%5D%2C%22buttonSets%22%3A%5B%22basic%2Bdiv%22%5D%2C%22functions%22%3A%5B%22f%22%2C%22g%22%2C%22h%22%5D%2C%22times%22%3Atrue%7D%2C%22version%22%3A%7B%22major%22%3A1%2C%22minor%22%3A0%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22itemDataVersion%22%3A%7B%22major%22%3A0%2C%22minor%22%3A1%7D%2C%22hints%22%3A%5B%7B%22content%22%3A%22%24%5C%5Cblue%7B%5C%5Ctext%7BThe%20sum%20of%20%2474%24%20and%20%242%24%7D%7D%24%20can%20be%20written%20as%20%24%5C%5Cblue%7B74%2B2%7D%24.%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22%24%5C%5Cpink%7B%5C%5Ctext%7B438%20minus%7D%20%5C%5Cblue%7B%5C%5Ctext%7B%20the%20sum%7D%7D%7D%24%20means%20that%20the%20entire%20sum%20is%20being%20subtracted%20from%20%24%5C%5Cpink%7B438%7D%24.%20%20%5Cn%5CnSo%2C%20we%20must%20put%20the%20expression%20%24%5C%5Cblue%7B74%2B2%7D%24%20in%20%5Cnparentheses%3A%20%20%5Cn%5Cn%24%5C%5Cqquad%28%20%5C%5Cblue%7B74%2B2%7D%29%24%5Cn%5CnThen%2C%20we%20put%20the%20%24%5C%5Cpink%7B438%7D%24%20outside%20of%20the%20parentheses%3A%5Cn%5Cn%24%5C%5Cqquad%5C%5Cpink%7B438-%7D%28%20%5C%5Cblue%7B74%2B2%7D%29%24%5Cn%5Cn%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22An%20expression%20that%20represents%20%24%5C%5Cpink%7B%5C%5Ctext%7B438%20minus%7D%20%5C%5Cblue%7B%5C%5Ctext%7B%20the%20sum%20of%2074%20and%202%7D%7D%7D%24%20is%20%20%24%5C%5Cpink%7B438-%20%7D%28%20%5C%5Cblue%7B74%2B2%7D%29%24.%20%20%5Cn%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%5D%7D)
answer the question and click score
verify that it says `correct: true`

Auditors: joel