nativeloader: Add CloseNativeLibrary() method

This method is called by art on UnloadNativeLibraries()

Bug: http://b/28406866
Change-Id: Ibccff93c83d6ccd3d0a1c0294a64b09281b4e05e
(cherry picked from commit 09a516bf16)

libnativeloader/include/nativeloader/native_loader.h

@@ -43,6 +43,9 @@ void* OpenNativeLibrary(JNIEnv* env,
                         jobject class_loader,
                         jstring library_path);
 
+__attribute__((visibility("default")))
+bool CloseNativeLibrary(void* handle);
+
 #if defined(__ANDROID__)
 // Look up linker namespace by class_loader. Returns nullptr if
 // there is no namespace associated with the class_loader.

libnativeloader/native_loader.cpp

@@ -231,6 +231,10 @@ void* OpenNativeLibrary(JNIEnv* env,
 #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);