Centers affordance options when wide.

When the layout is wider than the number of option items, the option
items appeared start-aligned. This CL makes them center-aligned instead.

Fix: 267802884
Test: manually verified that the items are center-aligned in landscape
mode on a large screen devicew and scroll normally in portrait mode,
when they don't all fit on the screen

Change-Id: I3299981a33c7d4acfc9831e05d02e4acc81b5ad9