nativeloader: Add CloseNativeLibrary() method

This method is called by art on UnloadNativeLibraries()

Bug: http://b/28406866
Change-Id: Ibccff93c83d6ccd3d0a1c0294a64b09281b4e05e
diff --git a/libnativeloader/native_loader.cpp b/libnativeloader/native_loader.cpp
index baad197..8cf140e 100644
--- a/libnativeloader/native_loader.cpp
+++ b/libnativeloader/native_loader.cpp
@@ -255,6 +255,10 @@
 #endif
 }
 
+bool CloseNativeLibrary(void* handle) {
+  return dlclose(handle) == 0;
+}
+
 #if defined(__ANDROID__)
 android_namespace_t* FindNamespaceByClassLoader(JNIEnv* env, jobject class_loader) {
   std::lock_guard<std::mutex> guard(g_namespaces_mutex);