Fixed "Correct Answer" lag in InteractiveGraph

Summary:
The correct answer text previously would show the correct answer for the
previous graph type selection until a point was moved. This commit fixes
this bug by refactoring the equationText out of this.state, making it
just part of what gets rendered. To do so, get*EquationString methods
all assume default values if this.props.graph.coords == null

Test Plan:
Visit test.html, add an interactivegraph widget, try switching between
all of the interactivegraph types, make sure Correct answer: always
displays text appropriate to the current selection.

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D4640