audiopolicy: make audio policy extensible

make function virtual or protected so that
they can be extended in custom audio policy.

audio policy: move output handle to AudioOutputDescriptor base class
Change-Id: I96ef9fd1c5a94874acb897245501ba2f9c9ab0c0

Conflicts:
	services/audiopolicy/managerdefault/AudioPolicyManager.h

Change-Id: Ida7992f6b327491fab1f4ea376e85e8eb34b89ca