audio HAL: add minimum supported version

Add a definition for the minimum audio HAL version supported
by the audio framework.

Change-Id: Id7df440cc9fcc23b27b61c81bacb569599a20c13
diff --git a/include/hardware/audio.h b/include/hardware/audio.h
index 6c28def..cbcb954 100644
--- a/include/hardware/audio.h
+++ b/include/hardware/audio.h
@@ -55,6 +55,8 @@
 #define AUDIO_DEVICE_API_VERSION_1_0 HARDWARE_DEVICE_API_VERSION(1, 0)
 #define AUDIO_DEVICE_API_VERSION_2_0 HARDWARE_DEVICE_API_VERSION(2, 0)
 #define AUDIO_DEVICE_API_VERSION_CURRENT AUDIO_DEVICE_API_VERSION_2_0
+/* Minimal audio HAL version supported by the audio framework */
+#define AUDIO_DEVICE_API_VERSION_MIN AUDIO_DEVICE_API_VERSION_2_0
 
 /**
  * List of known audio HAL modules. This is the base name of the audio HAL