Fix expression entry down arrow in Firefox

Previously, pressing down arrow was interpreted as a left parenthesis, since both have keyCode of 40 (and unlike Chrome and IE, Firefox sends keypress events for arrow keys). Arrow keys have charCode 0 so we'll instead just look for a charCode of 40 to detect parentheses.

Test Plan:
Tested open paren, close paren, backspace in Firefox and Chrome.

Auditors: alex