audio: Implement OnSessionStarted for HAL 2.1

Patch implements OnSessionStarted for HAL 2.1.
Bug:
Tag: #feature
Test: vts-tradefed run vts -m VtsHalBluetoothAudioV2_1TargetTest
Sponsor: jpawlowski@
Change-Id: Ide76fe59c323726ae98e6311f7360eb733c1c7cc

Change-Id: I84302cf653ffebcba3b4c875f6a156293d8dae75
diff --git a/bluetooth/audio/utils/session/BluetoothAudioSession.h b/bluetooth/audio/utils/session/BluetoothAudioSession.h
index 8d02387..83e20ad 100644
--- a/bluetooth/audio/utils/session/BluetoothAudioSession.h
+++ b/bluetooth/audio/utils/session/BluetoothAudioSession.h
@@ -79,6 +79,8 @@
 };
 
 class BluetoothAudioSession {
+  friend class BluetoothAudioSession_2_1;
+
  private:
   // using recursive_mutex to allow hwbinder to re-enter agian.
   std::recursive_mutex mutex_;