audio policy: implement routing control

Add implementation of audio routing control via AudioSystem APIs.

The following APIs are implemented:
- listAudioPorts(): return a list of devices and output/input mixers ports
that can be used as sources or sinks for audio patches.
- createAudioPatch()/releaseAudioPatch(): create/release a connection patch between
two audio ports (e.g. to connect input from an HDMI device to a speaker output device).
Only one client application can own a patch from a given source.
When an audio port (device or mix) is part of an application created patch, its routing cannot
not be changed by a policy decision.
- listAudioPatches(): return a list of existing patches.

Each audio port addition/removal and each audio patch creation/release increments a generation count.
This generation count is used to ensure consistency betwen calls to
listAudioPorts() and listAudioPatches().

Bug: 14815883.

Change-Id: I022b638c2f5f0bb41543c7cfca7488fb45cfdd80