Remove subclasses of NotificationEntryManager

First, moves most of the notification initialization logic out of
StatusBar and into NotificationController.

There is only one subclass of NotificationEntryManager in this project:

- CarNotificationEntryManager: stub implementation that disables most
functionality.

Removes CarNotificationEntryManager. Adds a config boolean to disable
notification rendering instead. Skips most notif initialization logic if
this flag is false.

Change-Id: I23ed86efa209f43314c162dcf32fe97112de7bc7
Test: atest, manual