docs: update template and css for android u

Add a custom version of docpage.cs to the droiddoc template, because
it adds a good deal of custom design and behavior (instead of updating
the doclava version of the file). Add CSS for next/previous links in
android u lessons and revise style for download button.

Change-Id: I829c4fb134d6f5c3cee82fb86958d23ef64e8481