Audio policy: implement configuration file in XML

This patch introduces configuration file of the policy in XML.
Legacy configuration file is used by default and
compilation flag USE_XML_AUDIO_POLICY_CONF must be defined to
use XML file format.

Change-Id: I352b788b5829d42ed82c851a4faef0e526be582e
Signed-off-by: François Gaffie <francois.gaffie@intel.com>