Audio routing: Fix notification routing during A11y playback

When notifications are played, the audio policy checks if media
  is playing for deciding which devices will play it. In the
  absence of media, the notification is duplicated on speaker
  and headset. With media playing, the notification routing
  is set to that of the media strategy.
The bug is observed during A11y playback on headset with a
  notification playing: A11y is playing on headset, but the
  device can't play the notification on speaker + headset at
  the same time, resulting in the notification playing in and
  out based on the A11y playback in headset.
The fix is to also consider A11y activity when routing
  notification playback (under STRATEGY_SONIFICATION_RESPECTFUL).
  This improves the UX the notification is not missed, and
  doesn't cut in and out.

Test: play Talkback on headset, receive notification at the same time.
Bug: 62907561
Change-Id: Ib924ec41cb7e16d6f2686d8615facba08e256ade