network-flow(donut): open the remainder from the ring and the legend

The chart has emitted an activated slice since the ring first had a
highlight, and nothing listened. Now the widget does, for the one slice that
has anything behind it: the arc and the legend's chevron both reach the same
breakdown.

Which also settles what the other arcs were claiming. Every slice was a
button and every slice was tabbable, so a reader working through the ring
with a screen reader was offered eight controls of which one did anything.
Drillability now decides the affordance, the way the legend has always only
drawn its chevron on the remainder, and a slice that opens nothing is a
shape again rather than a button that does not answer.

The panel is asked about the window the ring's numbers came from, and that
window travels with the slices rather than beside them: the fetch discards a
response that arrived out of order, and a window kept alongside would have
survived that discard and pinned the panel to a ring nobody saw.

Clicking an arc also focuses it first. A pointer does not focus an SVG group
by itself everywhere, and without that the panel has nowhere to hand the
keyboard back to when it closes - which is invisible until someone tries it
with a screen reader.

CMK-37878

Change-Id: I7cc3513eb7fd5cb49841b9cc741c6fef900a0e6d