cpp: use proper nativehelper headers

libnativehelper exports headers under nativehelper. These were
available before incorrectly as global headers in order to give
access to jni.h.

Test: modules using cpp find headers
Bug: 63762847
Change-Id: I34722bb961327b0128703ab925bc86961ca66cb6
diff --git a/media/jni/android_media_MediaMetricsJNI.cpp b/media/jni/android_media_MediaMetricsJNI.cpp
index 8979cec..38f7a7e 100644
--- a/media/jni/android_media_MediaMetricsJNI.cpp
+++ b/media/jni/android_media_MediaMetricsJNI.cpp
@@ -16,7 +16,7 @@
 
 #include <android_runtime/AndroidRuntime.h>
 #include <jni.h>
-#include <JNIHelp.h>
+#include <nativehelper/JNIHelp.h>
 
 #include "android_media_MediaMetricsJNI.h"
 #include <media/MediaAnalyticsItem.h>