applets/notifications: Improve header text contrast

The app name in the header has poor text contrast because it is 40%
transparent and sits on a gray background. It is also using a
semantically incorrect UI component (DescriptiveLabel) which was meant
for de-emphasized subtitles, not titles. The correct component for that
is a Heading, so let's use that.

Because this means that we will hve two identically-sized headings in
close proximity, we need to distinguish the text for the notification
title without making it huge, which is overkill for a notification. To
accomplish this, this MR uses the Heading's "Primary" type which was
made for this purpose.