audio policy: remove deserialization from policy objects

This patch removes the responsability of the deserialization of all
objects managed by the policy by:
-adding required accessors
-moving deserialization function to Config parser helper
-adds a TypeConverter to associate string to / from values of all
audio enumeration shared between HALs and policy.

Change-Id: I1ce798848f4657b37e47446c9fbdc63f7ed0390e
Signed-off-by: François Gaffie <francois.gaffie@intel.com>