audiopolicy: exclusive Preferred Device

It allows to force the routing of music during call without
affecting clients that does not explicitely requested to use
another device than the one decided by the policy during call
(which is TELEPHONY TX).

It fixes differently Bug 111467967.

Test: make

Change-Id: I6034f0d2568e1b2a1e600d9ae1453fd0c60ed02e
Signed-off-by: François Gaffie <francois.gaffie@renault.com>