wifi(implementation): Add global lock

Add a global lock to address synchronization issues between the main
HIDL method servicing thread and the legacy HAL's event loop thread.

Also, added some documentation for the threading model used.

Bug: 34261034
Test: Compiles
Change-Id: I881111814ff5ebd601d6a4c85cf284b30ae47ed3
diff --git a/wifi/1.0/default/hidl_sync_util.cpp b/wifi/1.0/default/hidl_sync_util.cpp
new file mode 100644
index 0000000..7d47f2f
--- /dev/null
+++ b/wifi/1.0/default/hidl_sync_util.cpp
@@ -0,0 +1,39 @@
+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "hidl_sync_util.h"
+
+namespace {
+std::recursive_mutex g_mutex;
+}  // namespace
+
+namespace android {
+namespace hardware {
+namespace wifi {
+namespace V1_0 {
+namespace implementation {
+namespace hidl_sync_util {
+
+std::unique_lock<std::recursive_mutex> acquireGlobalLock() {
+  return std::unique_lock<std::recursive_mutex>{g_mutex};
+}
+
+}  // namespace hidl_sync_util
+}  // namespace implementation
+}  // namespace V1_0
+}  // namespace wifi
+}  // namespace hardware
+}  // namespace android