AudioPolicyManager: distinguish between mirroring and rerouting

In the screen mirroring case, the notification manager queries
 the policy to see if media "is playing remotely": if yes, it
 will not request audio focus before playing a notification.
This is the correct behavior for screen mirroring, but doesn't
 work when audio is rerouted to an external policy.
This change consists in not reporting "remote" activity when
 playing to a mix of an external policy by looking at the address
 of the output on which playback is active for the given stream
 type.

Bug 16009464

Change-Id: Iebbf5a8c738ab36c1cf2055e921c0a3472503125