Using ImageResource solves two problems:

1) Listeners can be notified synchronously when the sky.Image is already
   available. This change removes flash of 0x0 layout when moving an
   already-cached image around in the render tree.

2) In the future, when we support animated images, we can notify listeners
   multiple times whenever a new image is available.

TBR=abarth@google.com

This patch contains some minor improvements to the network service:

- We now cache the connection to the network service so we don't need to spin
  it up for each request.
- We now manage the lifetime of NetworkServiceImpl in the same way as other
  services.

Also, update the workbench sky_tools dependency to the latest version.