audio: Add constant for MSD HAL service name

The constant is used both by the MSD HAL service to register
itself, and by audioserver in order to discover it.

Bug: 63901775
Test: build
Change-Id: I78a513de501dda1bbd6742298af78de485c7b325
diff --git a/audio/include/system/audio.h b/audio/include/system/audio.h
index e683d60..1c11add 100644
--- a/audio/include/system/audio.h
+++ b/audio/include/system/audio.h
@@ -984,6 +984,20 @@
 #define AUDIO_HARDWARE_MODULE_ID_STUB "stub"
 
 /**
+ * Multi-Stream Decoder (MSD) HAL service name. MSD HAL is used to mix
+ * encoded streams together with PCM streams, producing re-encoded
+ * streams or PCM streams.
+ *
+ * The service must register itself using this name, and audioserver
+ * tries to instantiate a device factory using this name as well.
+ * Note that the HIDL implementation library file name *must* have the
+ * suffix "msd" in order to be picked up by HIDL that is:
+ *
+ *   android.hardware.audio@x.x-implmsd.so
+ */
+#define AUDIO_HAL_SERVICE_NAME_MSD "msd"
+
+/**
  * Parameter definitions.
  * Note that in the framework code it's recommended to use AudioParameter.h
  * instead of these preprocessor defines, and for sure avoid just copying