hal: Add DYNAMIC_VOLUME_MIXER ifdef for AOSP compatibility

 * Upstream kernel uses a much newer compress offload implementation
   which supports gapless playback, multiple streams, and effects.
 * The volume for these streams can be controlled independently,
   so add an ifdef if we support the new mixer names.

Change-Id: I6633ec5c95ba8a8e4424be82fe90fe207a66b964