network-flow(donut): fade the center from one reading to the next

Pointing at a slice swapped the text in the hole of the ring outright. At the
speed a pointer crosses a ring that reads as a flicker, and the reader is left
to notice for themselves that the number now means something else.

The reading is keyed on what it is about and stacked in its own layer, so the
outgoing and the incoming one are both on screen while one takes over from the
other. A refresh that leaves the reading about the same thing does not
re-enter it. Under prefers-reduced-motion the fade collapses to nothing.

Change-Id: Ia08a5964840b7dd5850e685d7b486c174ff665c7