New default IClientCallback implementation

Add a default IClientCallback implementation which exits once all
watched services have no more clients. This is useful for lazy HALs.

Test: Use DefaultCounterExitCallback to register a HAL that implements
      two interfaces and check that it exits when both interfaces are
      unused.
Change-Id: Ifcc53631e284375a436d618e84e9546a5e53844a
diff --git a/transport/include/hidl/HidlLazyUtils.h b/transport/include/hidl/HidlLazyUtils.h
new file mode 100644
index 0000000..fc9c268
--- /dev/null
+++ b/transport/include/hidl/HidlLazyUtils.h
@@ -0,0 +1,41 @@
+/*
+ * Copyright (C) 2018 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.
+ */
+
+#pragma once
+
+#include <android/hidl/base/1.0/IBase.h>
+#include <utils/RefBase.h>
+#include <utils/StrongPointer.h>
+
+namespace android {
+namespace hardware {
+namespace details {
+class LazyServiceRegistrarImpl;
+}  // namespace details
+
+/** Exits when all HALs registered through this object have 0 clients */
+class LazyServiceRegistrar {
+   public:
+    LazyServiceRegistrar();
+    status_t registerServiceWithCallback(const sp<::android::hidl::base::V1_0::IBase>& service,
+                                         const std::string& name);
+
+   private:
+    std::shared_ptr<details::LazyServiceRegistrarImpl> mImpl;
+};
+
+}  // namespace hardware
+}  // namespace android