AudioPolicyManager: fix closure of output supporting more than on device

When an output supports more than one device which is considered
as "distinguished on address", if one of these devices is closed,
the output is closed.

This CL fixes this behavior by adding a guard on no more available
device is supported by the given sw output.

Signed-off-by: Francois Gaffie <francois.gaffie@renault.com>
Change-Id: Ibc6da5a8b0f82c0f3f4ce33f781bfd84e597ac0f