Follow O patterns for a11y settings

Move the summary for the toggled features to a footer.
Make the settings appear as a preference.

Bug: 36780887
Test: Start settings and observe the new UI looks a lot
like the mocks referenced in the bug. Also added unit
test to verify that summary text shows up.

Change-Id: I1d002b194991d0901ecb27198ba5de73bd23a5a9