audio policy: parse device descriptors in config file

Implement parsing of audio_policy.conf for device and gain
controller definitions.
Copy audio_policy_conf.h from hardware_legacy.
New syntax for devices and gain controllers description will not
be parsed by legacy audio policy manager.

Bug: 14815883.

Change-Id: I7f1035d514dcf55fb3e45ed1f633a2f63ee398f5