Properly center and scale the navkey glow.

The overall scale of the glow is currently based on the
height of the navigation bar. That is, the glow asset will
be scaled so its height fits perfectly in the bar when at
rest (i.e. not enlarged by active touch).

Bug: 6399510
Change-Id: I95139787a435bcf69d5fda0003c84e4fe55c330d