audio policy: move device_category type outside volume class

This patch prepares the removal of hard coded volume tables by
moving outside volume class the device_category enumeration.

Change-Id: Ic712377898276a39bb782ac0bc7f793cdd7fa8bd
Signed-off-by: François Gaffie <francois.gaffie@intel.com>
diff --git a/services/audiopolicy/engine/interface/AudioPolicyManagerInterface.h b/services/audiopolicy/engine/interface/AudioPolicyManagerInterface.h
index e73e543..3a3bdbf 100755
--- a/services/audiopolicy/engine/interface/AudioPolicyManagerInterface.h
+++ b/services/audiopolicy/engine/interface/AudioPolicyManagerInterface.h
@@ -143,7 +143,7 @@
      *
      * @return amplification value in dB matching the UI index for this given device and stream.
      */
-    virtual float volIndexToDb(Volume::device_category deviceCategory, audio_stream_type_t stream,
+    virtual float volIndexToDb(device_category deviceCategory, audio_stream_type_t stream,
                                  int indexInUi) = 0;
 
     /**