Refactor NetworkNotificationManager.

1. Move from deprecated network types to transport types.
2. Rename and simplify (by passing in a NetworkAgentInfo object)
   the call signature of the method that displays notifications.
3. Add a method to clear notification, and unindent lots of code.
4. Move the legacy DcTracker-issued notification code to
   NetworkNotificationManager.

Bug: 31025214
Change-Id: Ie49c60126d0ed5bac620bc47e84fe038791b2d6c