Expand hit rectangles of main navigation buttons.

There's now no dead space between them. Also, increase the
visual size of the nav icons by 30% (just a quick resample
of the current artwork, to be replaced by custom-drawn
graphics at 130%).

The background glow looks kind of comical right now.

Bug: 6381833
Change-Id: I03ef0ea64a9e953f9f2b29cc03097d250ff52134