While at it, also removed HeaderHtml.soy since it's anyway not used
and it's strange that there is no Header.soy. Once we start using
more headers, it may be needed to bring Header.soy back and do
similar logic to Footer.soy to loop over all the footers/headers
instead of adding them in format() as done now.
Ensured the attention set header is only visible if the attention
set is non-empty, and if the attention set is enabled.
Change-Id: I805c625e063ee56ba71588dd684b34cd5c318290
10f035a906