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.cpp b/media/libaudiohal/impl/StreamHalHidl.cpp
index f4e93b5..e162a68 100644
--- a/media/libaudiohal/impl/StreamHalHidl.cpp
+++ b/media/libaudiohal/impl/StreamHalHidl.cpp
@@ -931,6 +931,11 @@
     callback->onRecommendedLatencyModeChanged(modes);
 }
 
+status_t StreamOutHalHidl::exit() {
+    // FIXME this is using hard-coded strings but in the future, this functionality will be
+    //       converted to use audio HAL extensions required to support tunneling
+    return setParameters(String8("exiting=1"));
+}
 
 StreamInHalHidl::StreamInHalHidl(
         const sp<::android::hardware::audio::CPP_VERSION::IStreamIn>& stream)