Update SwapUpdater to use computed shortest path length instead of Manhattan distance

[weinstein]

The current SwapUpdater implementation (#146) takes a short cut and uses manhattan distance when computing the shortest GridQubit-to-GridQubit distances.

But the manhattan distance is only the same as the shortest path length on the device connectivity graph when there are no unusable qubits (no holes, hanging strips, or disconnected qubits).

We should update the SwapUpdater to precompute shortest path lengths on the device connectivity graph (ex using the Floyd-Warshall algorithm, which we're already using for the initial circuit placement) and use those distances instead of manhattan distance. Otherwise, circuit placement will break in the presence of pruned qubits (see also quantumlib/unitary#51).