Zen QS: Tweak segmented button + introduction styling.

 - Add a rounded rect background to the segmented buttons.
 - Remove the spacing between the segmented buttons.
 - Increase the minheight + font weight of the button text.
 - Remove the embedded divider line.
 - Add a rounded rect background to the intro messages.
 - Tweak margins, mostly 8dp -> 16dp in a few places.

Bug: 20949290
Bug: 20947407
Change-Id: Ic497c772aa7228423c8884d6ec7b9ccbfdda365a