network-flow(donut): let the number in the ring scale with the ring

The volume in the middle of the donut was sized off the widget's text size,
which has nothing to do with how much room the hole actually has. Zoom in,
or drop the widget into a narrow tile, and the ring shrinks while the number
does not: "40.69 GB" ends up wider than the hole it is supposed to sit in,
printed across the arcs it is meant to be surrounded by.

The figure becomes a container of its own, so the center text is measured
against the ring instead. It is also held to 70% of the figure's width -
the hole is a little under three quarters of it across - and truncates there
rather than reaching into the arcs, for the one case a proportional size
cannot cover: a long enough formatted value at the smallest ring a dashboard
will draw.

CMK-37876

Change-Id: Id96df856339f8f0e5d1eae5979d0f34220386091