stagefright: Fix compile for non-QCOM_HARDWARE

* AUDIO_FORMAT_FLAC is only defined for QCOM_HARDWARE, move it inside
  the AV_ENHANCEMENTS ifdef so only AV_ENHANCEMENTS enabled devices can
  possibly offload it.

Change-Id: I83c1d527527c2545c5cea599fb01632604c73c23
Merge "stagefright: Fix compile for non-QCOM_HARDWARE" into du44