oauth2: don't use http.DefaultTransport.

http.DefaultTransport is not available on App Engine.
App Engine Managed VMs should depend on url fetcher.
Revert "App Engine Managed VMs should depend on url fetcher."

This reverts commit 4901e89da79a1d4f2d0d53e762de98c66428f21d.
Use default client rather than a new client instance.
Export Transport to allow default transport to be configurable.