Notifications are not duplicated over multiple devices

Sonifications follow media routing, except for speaker playback
  where they follow the "safe" path when available in the
  absence of media playback.

Test: 1/ plug headphones on phone, play notification, verify it
         comes only from headset
      2/ mirror device and play media, then notification, verify
         it only comes from the speaker

Change-Id: I762ad0688e879ad1e5565b4b77a7393fa7ce7ac5