audio: add flag to only set min buffersize instead of min and max

Sony MSM8960 devices expect to only get min buffersize instead of
min and max size.

Change-Id: I8e4db2b32592ebe5a2945bf2b3c311f2467f8814
audio: Don't build if TARGET_QCOM_AUDIO_VARIANT is set

 * To allow us to provide the Qualcomm enhanced audio HAL for
   devices that want to use it.

Change-Id: I01f8241eb1137d5937dace32d0859484a3d31fcf