More AudioPolicy registration

Define new usage in AudioAttributes to describe "virtual" sources.
Maintain a registration string in AudioPolicy and AudioMix that
 is used for the address of the mix in the native policy manager.
AudioPolicy registration marks the audio policy configuration and
 the audio mixes as registered.
The AudioPolicy class is used as the factory for the AudioTrack
 and AudioRecord objects that are respectively used as the
 source and sink of the mixes created by the policy owner.

Bug 16009464

Change-Id: I8e5bd351495257597c513d22597e273fcabdbc55