Prevent notifications from muting accessibility prompts

When evaluating on which device a sound is to be played, the policy
  must consider which current routing strategy, if any, has priority
  for overriding the choice. Here the playback of notifications,
  when duplicated over speaker and headphones, was causing the
  accessibility prompt to be muted because the it was of a higher
  priority, and incompatible with the accessibility routing.
The fix consists in assigning a higher priority to the accessbility
  routing strategy over the notification routing strategy.

Bug 18834451

Change-Id: I8228b30a7d80bd61d1c223afb030d9421d4f33cf