hal: check out_snd_device to get a2dp sample rate

There could be one usecase which:
  - there're two active streams in AHAL playing on
    BT_SCO device;
  - then if one of the active stream receives 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 BT_SCO;
  - so the app sample rate get for this output is wrong.
Instead of checking out device, check snd_device for
querying app sample rate.

Change-Id: Icb7dbba8dc24ee8e5611b1e813e0e4aa2aa3f990