frameworks/base: 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 frameworks/base find headers
Bug: 63762847
Change-Id: I0f9f231acdebe460f279135462f43d3e32eff64d
diff --git a/core/jni/android_media_ToneGenerator.cpp b/core/jni/android_media_ToneGenerator.cpp
index aec6263..ecb3cd6 100644
--- a/core/jni/android_media_ToneGenerator.cpp
+++ b/core/jni/android_media_ToneGenerator.cpp
@@ -22,7 +22,7 @@
 #include <fcntl.h>
 
 #include <jni.h>
-#include <JNIHelp.h>
+#include <nativehelper/JNIHelp.h>
 #include "core_jni_helpers.h"
 
 #include <utils/Log.h>