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_hardware_SerialPort.cpp b/core/jni/android_hardware_SerialPort.cpp
index 393dc7b..190ddce 100644
--- a/core/jni/android_hardware_SerialPort.cpp
+++ b/core/jni/android_hardware_SerialPort.cpp
@@ -19,7 +19,7 @@
 #include "utils/Log.h"
 
 #include "jni.h"
-#include "JNIHelp.h"
+#include <nativehelper/JNIHelp.h>
 #include "core_jni_helpers.h"
 
 #include <stdio.h>