audiopolicy: apm: Fix default bus device issue

When a bus device is used as default, when calling setForceUse,
a given SwOutputDescriptor might be asked to switch to another
bus device (the default), violating the Policy Mix rule.

Test: see below
 -start an automotive device with PolicyMix registered and
  bus device as the default.
 -Consider having a system bus for system sound and media for music.
 -Play touch sound (check played in system bus).
 -launch music player.

Result:
 -Music shall be played on media bus device
 -Touch sound still played on system bus device

Change-Id: I6668cffa525af51af6ed0838745658f72ae00a62
Signed-off-by: François Gaffie <francois.gaffie@renault.com>