interactive-number-line: fix open circle

Summary:
KhanUtil.BACKGROUND is now KhanUtil._BACKGROUND; this commit
updates interactive-number-line to use KhanUtil._BACKGROUND.

Test plan:
 * Open http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5B%5B%E2%98%83%20interactive-number-line%201%5D%5D%22%2C%22widgets%22%3A%7B%22interactive-number-line%201%22%3A%7B%22type%22%3A%22interactive-number-line%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22range%22%3A%5B0%2C10%5D%2C%22labelStyle%22%3A%22decimal%22%2C%22labelTicks%22%3Afalse%2C%22tickStep%22%3A1%2C%22snapDivisions%22%3A4%2C%22correctRel%22%3A%22lt%22%2C%22isInequality%22%3Atrue%2C%22correctX%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%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22hints%22%3A%5B%5D%7D
 * Click "Make circle open"
 * Verify that the circle becomes open

Auditors: eater, marcia