audiopolicy: Add notification when default effects are updated

 * In M, we now have the ability to define a default set of audio effect
   on a per-stream basis. This allows us to get around the problem of
   apps not sending the control intents so we can implement smart
   global effects for specific media types.
 * We still need a session id in order to get a handle and configure them
   from an app like AudioFX, so we'll need to add some plumbing in
   order to send an event to interested applications.
 * This patch implements the native side of this. The Java layer will
   call down thru AudioSystem and register a callback which will be
   invoked by the audio policy when default effects are updated on
   a stream. This callback will receive both the stream type as well
   as the session id.
 * Attaching this listener requires that the caller hold the
   MODIFY_AUDIO_ROUTING permission.

Change-Id: I142b15f2585ffca6a953c3e828e2a7c07b24f56c