NetworkNotificationManager gets NotificationManager locally

It's confusing that NetworkNotificationManager uses
NotificationManager to put up notifcation which is passed from
ConnectivityService. And that NotificationManager should be
gotten from a context with UserHandle.ALL because the
notification should notify to all users. But the future developer
may not know this and pass incorrect NotificationManager
probably. Therefore, adjust the design that
NetworkNotificationManager will get NotificationManager locally.

Bug: 170593746
Test: atest FrameworksNetTests
Change-Id: Id65c998052b7b223171c5194ea46b49d7e84729d