Bug 881863 - [Gaia] B2G CDMA UI support for emergency call back mode

This Emergency Callback Mode Manager is for:
 - Initialize the manager if network type is cdma and regist an event handler for emergency callback mode change.
 - Display a notification to showing the system is entered the callback mode with count down timer.
 - Popup a dialog for user to notify the mode will block data network and let user decide callback mode should exit or not.