Add AudioMix and AudioMixCollection to common policy element

This patch adds AudioMix and associated collection to the common
policy pillar elements library.
It removes the code from the manager and creates also helpers function
associated to the mix collection.

Change-Id: Ia3536fdef09da13734d2895030d736c701040237
Signed-off-by: François Gaffie <francois.gaffie@intel.com>