network-flow(donut): let the keyboard reach every slice, not just the last one

Only the aggregated remainder carried a role and a tab stop, because it was
the only slice with a panel behind it. That left the rest of the ring
unreachable without a pointer, and their aria-labels unread: a bare <g> with
no role is not a thing assistive technology stops at. It also quietly broke
the focus half of the highlight, since focus events never reached a group
nothing could focus.

Every slice is a button now, and activation is reported for all of them. What
an activation opens stays the caller's decision -- the widget still only
opens the breakdown for the remainder -- so the ring gains no knowledge of
what is behind any slice. A collapsing slice remains out of reach.

Change-Id: Ia7fcc89ecb76683453f5d986e0d9b7cbe47ea2e6