Make exercise variable names consistent

Summary: Changes several variable names: the global `exerciseId`, `exerciseName` and `exerciseFile` as well as `problemId` and `problemSeed` were prepended with "current" (`currentExeriseId`, `currentExerciseName`, `currentExerciseFile`, `currentProblemType`,`currentProblemSeed`), `id` in one location (`loadExercise()`) was renamed to `exerciseId` and in another (`makeProblem()`) to `typeOverride`, and `seed` to `seedOverride`

Test Plan: Tried in absolute_value (has no type names), a perseus exercise (recognizing rays; not expected to work), and range_of_a_function (with type names) with functions Khan.getPreviewUrl() and Khan.getSeedInfo().

Reviewers: alpert

Reviewed By: alpert

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6160