Allow user to specify whether a new network is shared or private

* Add a check box to the WiFi configuration dialog that lets the user
  choose whether a newly created configuration is shared with other
  users (defaults to true)
* Disable the check box when modifying an existing network

BUG=25600871

Change-Id: Ifc6713602ee61b0407e55f45097c1b311fa19cb4
3 files changed