Add hasEnabledNotificationListener API

This CL adds a system API to NotificationManager for checking whether
the NotificationListener for the current package name and user id is
enabled or not.

This replaces existing implementation which was using binder calls
directly to NotificationManagerService.

Bug: 160762852
Test: atest CtsMediaHostTestCases:\
    android.media.session.cts.MediaSessionManagerHostTest\
    #testGetActiveSessions_withSession2

-> MediaSessionManager#getActiveSessions verifies call by checking if
the notification listener is currently enabled.

Change-Id: Ic730b0abfe08c40171062421b2015be9cc05310a