a2dp: check out_snd_device to tear down a2dp path

During a2dp switching to sco, by expected, the active
a2dp paths should be teared down before sco connection
established to avoid trailing data sent over slimbus.
There could be one usecase which:
  - there're two active streams in AHAL, while one of
    them is stopped in AudioPolicyManager.
  - then only one active stream can receive param
    "routing=a2dp" and switch path to a2dp, the other one
    will also be switched to a2dp as part of
    check_usecases_codec_backend, but the out device is
    still the stale device.
  - after A2dpSuspended=true is received, only the stream
    with out device is a2dp can be teared down, the other
    one can keep a2dp active for a while.
Instead of checking out device, check snd_device for tearing
down active streams on a2dp device.

Change-Id: Id7186f312d4aff10b59b6619be5c761f9d592e9b