audio policy: session route: move SessionRoute to common

Move the SessionRoute and SessionRouteMap inner class from policy
manager to common policy pillars as initiated by the refactor.

This code was added at teh wrong place by commit aa981194.

Bug: 22376521.

Change-Id: I29aca21614e409c59aae4b7b14163552fb0bffb1
Signed-off-by: François Gaffie <francois.gaffie@intel.com>