Combines NotificationEntryManager listener interfaces (part 2).

This change introduces the NotificationAlertingManager component, a
NotificationEntryListener which handles showing/hiding/updating alerts
(heads-up or ambient pulsing) in response to events from the
NotificationEntryManager.  All code in the new component was moved out
of NotificationEntryManager proper.

Test: atest SystemUITests, manual
Change-Id: I46a046da6caf39b1d314b357e21ac6b4755c5796