Mark 'session' field in audio HAL AIDL as unused.

The 'session' field is only used by the framework (if at all),
so it being moved to the HAL AIDL to the system-specific part.

Also update the comment on AudioPort.

Bug: 205884982
Test: m
Change-Id: Ia214c343bbf08c7944ea5ebf6140a92821941b46

media/aidl/android/media/audio/common/AudioPort.aidl

@@ -23,8 +23,9 @@ import android.media.audio.common.AudioProfile;
 import android.media.audio.common.ExtraAudioDescriptor;
 
 /**
- * Audio port structure describes the capabilities of an audio port
- * as well as its current configuration.
+ * Audio port structure describes the capabilities of an audio port.
+ * This is a "blueprint" which contains all the possible configurations
+ * that are supported by the port.
  *
  * {@hide}
  */

media/aidl/android/media/audio/common/AudioPortExt.aidl

@@ -34,6 +34,9 @@ union AudioPortExt {
     AudioPortDeviceExt device;
     /** Information specific to mix ports. */
     AudioPortMixExt mix;
-    /** Audio session identifier. */
+    /**
+     * NOT USED. Framework audio session identifier.
+     * Use android.media.AudioPortExtSys.session on the system side.
+     */
     int session;
 }