Remove explicit use of the "exiting" parameter from AudioFlinger

Pushed into libaudiohal to establish a cleaner abstraction and allow
for a strongly-typed interface in the future.

Test: Manual verification of basic audio use-cases
Change-Id: I1eb2bd2013f7129e65fe8a672ac4305f928cc9ef
diff --git a/media/libaudiohal/impl/StreamHalHidl.h b/media/libaudiohal/impl/StreamHalHidl.h
index 21b0627..1dbdf4c 100644
--- a/media/libaudiohal/impl/StreamHalHidl.h
+++ b/media/libaudiohal/impl/StreamHalHidl.h
@@ -202,6 +202,8 @@
 
     void onRecommendedLatencyModeChanged(const std::vector<audio_latency_mode_t>& modes);
 
+    status_t exit() override;
+
   private:
     friend class DeviceHalHidl;
     typedef MessageQueue<WriteCommand, hardware::kSynchronizedReadWrite> CommandMQ;