Removes circular dependency between NotificationMediaManager and NotificationEntryManager.

This change makes NotificationMediaManager register itself as a
NotificationEntryListener instead of having NotificationEntryManager
separately and explicitly notify it of a notification being removed.
This allows us to remove the dependency of NotificationEntryManager on
NotificationMediaManager.  (The reverse dependency already existed.)

Test: atest SystemUITests, manual
Change-Id: I0577bf864668ccbecd70a3c3b13b610f981f7fc4