Restructure public side of network service.

This CL goes from this:
  //mojo/services/public/cpp/network
  //mojo/services/public/interfaces/network

to this:
  //mojo/services/network/public/cpp
  //mojo/services/network/public/interfaces

This CL also makes the Mojo-side changes required to roll this change into
Chromium.

TBR=beng

Review URL: https://codereview.chromium.org/789243002