hal: add support for external speaker driver library

Bug:  17319721

Conflicts:
	hal/audio_extn/audio_extn.h
	hal/audio_hw.c
	hal/audio_hw.h

Change-Id: Ia3e96c2af7e4e83e45e121b63343d7f6040af060
diff --git a/hal/audio_hw.h b/hal/audio_hw.h
index aa866a5..0394497 100644
--- a/hal/audio_hw.h
+++ b/hal/audio_hw.h
@@ -262,6 +262,8 @@
     unsigned int cur_codec_backend_bit_width;
     void *platform;
     unsigned int offload_usecases_state;
+    void *extspk;
+
     void *visualizer_lib;
     int (*visualizer_start_output)(audio_io_handle_t, int);
     int (*visualizer_stop_output)(audio_io_handle_t, int);