Show notifications after capport login

Add NetworkStackNotifier to manage notifications created by
NetworkStack.
NetworkStackNotifier handles the "connected" notifications shown after
connecting to a captive portal, which may contain information sourced
from the captive portal API.

Test: atest NetworkStackTests
Bug: 139269711
Change-Id: Iaf96f7e5f02be04b098230316595ad4c0777a9d8