Add press states for dropdown in perseus

Summary:
Screenshots coming soon!

then:
{F58246,size=full}

now:
{F58245,size=full}

Test Plan: Go to an exercise like "Math Patterns 2" ("math-patterns"), and look at how tapping a dropdown item feels livelier.

Reviewers: marcos, mgp

Reviewed By: mgp

Subscribers: aria

Differential Revision: http://phabricator.khanacademy.org/D13242