audio policy: enhancing type conversion helper

This patch updates the conversion helper class (string to/from policy
common types) by adding direction agnostic helper for channels and
formats.
It also adds the support of device category and stream type parsing.

Change-Id: I75d5a77d78bc3aeb2389b424bfb41febbf020f1a
Signed-off-by: François Gaffie <francois.gaffie@intel.com>