audio policy service: Add introspection API to retrieve AudioProductStrategies

This CL adds required introspection APIs to deal with product strategies:

-getter of the collection of product strategies
-helper function to return the strategy associated to a given attributes.
This API is mandatory to avoid duplicating the logic that compiles the strategy
for a given Audio Attributes structure.

Test: make

Change-Id: I0e107570a44227bb52a4f359954c93215d4f8bae
Signed-off-by: François Gaffie <francois.gaffie@renault.com>