Popup a notification after logging in the captive portal network

Captive portal app will be auto dismissed after user login the
captive portal network. In order to improve the user experience,
popup a notification to notify user that the captive portal
network is connected.

Bug: 113629026
Test: 1.atest FrameworksNetTests:NetworkNotificationManagerTest
      2.Connect to a captive portal network and login, check if
      there is a notification popup.

Change-Id: Id54d12268e107af2f213c2bb348c5f7908e880f4