Add padding above and below dialog lists to match Material spec

There's a little too much padding between lists and buttons, but it's
definitely the lesser of two evils. We can handle it dynamically later
on, but it's tricky since we can't just insert a Space element.

BUG: 18336555
Change-Id: I1a8ff644dd27f7998ce7cf7a4fadba1fd7ed04ff