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_MediaExtractor.cpp b/media/jni/android_media_MediaExtractor.cpp
index c9657b1..d2fa8f5 100644
--- a/media/jni/android_media_MediaExtractor.cpp
+++ b/media/jni/android_media_MediaExtractor.cpp
@@ -27,7 +27,7 @@
 #include "android_runtime/Log.h"
 #include "android_util_Binder.h"
 #include "jni.h"
-#include "JNIHelp.h"
+#include <nativehelper/JNIHelp.h>
 
 #include <android/hardware/cas/1.0/BpHwCas.h>
 #include <android/hardware/cas/1.0/BnHwCas.h>