NotificationManagerService: don't play notif in SILENT mode

Fix the test for playing a notification sound:
  play sound when 1/ no exclusive focus
             AND  2/ volume not 0
Previous test would play a notification if device was in SILENT
  mode (volume was 0, but ringer mode was != VIBRATE)

Bug: 75043398
Test: enter silent mode, play notification, verify no ducking
Change-Id: I0e19d068f719a89c7a8a3c239da86c1dfce701ec