Improved performance of Fancy Dropdown animations

Summary:
Fancy Dropdowns now animate with `translate`, rather than adjusting `margin`,
which invalidated styles and layout, and wasn't very performant, especially on older
devices (e.g., iPad 3). These new animations emulate the old behavior, but by using
`transform`, we save the browser a lot of work, leading to a silky smooth, Andy-verified
effect.

Test Plan:
- Run the mobile app.
- Go to "Measuring lengths with different units".
- Hit the dropdowns a bunch of times. Hope for the best!

Reviewers: jack, alpert

Reviewed By: alpert

Subscribers: alpert, andy

Differential Revision: http://phabricator.khanacademy.org/D11986