network-flow(donut): move the change text where both readers can reach it

The legend works out what a change against the previous period reads as -
the sign, the "new" for growth out of nothing, the dash for nothing to
compare. The breakdown of the aggregated remainder needs the same column,
and a second copy of those three rules is a second chance to word them
differently: two tables in the same widget disagreeing about what "no
history" looks like.

So it moves next to the byte and count formatters, where the widget's other
shared renderings already live, along with the dash itself - the legend was
the only place that knew what an absent number looks like, and now it is not.

CMK-37878

Change-Id: I20de61f32f0d707a98e2ff7821dd1deba09459ee