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_util_AssetManager.cpp b/core/jni/android_util_AssetManager.cpp
index 171bd89..15966ad 100644
--- a/core/jni/android_util_AssetManager.cpp
+++ b/core/jni/android_util_AssetManager.cpp
@@ -37,9 +37,9 @@
 #include "android_util_Binder.h"
 #include "core_jni_helpers.h"
 #include "jni.h"
-#include "JNIHelp.h"
-#include "ScopedStringChars.h"
-#include "ScopedUtfChars.h"
+#include <nativehelper/JNIHelp.h>
+#include <nativehelper/ScopedStringChars.h>
+#include <nativehelper/ScopedUtfChars.h>
 #include "utils/Log.h"
 #include "utils/misc.h"
 #include "utils/String8.h"