graphics: convert composer default impl to a library

Convert composer default impl to a header-only library,
android.hardware.graphics.composer@2.1-passthrough.

Test: builds and VTS
Change-Id: I9251aadc28816fc4c1d9326e09e297f30e9c25fe
diff --git a/graphics/composer/2.1/default/passthrough.cpp b/graphics/composer/2.1/default/passthrough.cpp
new file mode 100644
index 0000000..ef7ed7c
--- /dev/null
+++ b/graphics/composer/2.1/default/passthrough.cpp
@@ -0,0 +1,25 @@
+/*
+ * Copyright 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 <android/hardware/graphics/composer/2.1/IComposer.h>
+#include <composer-passthrough/2.1/HwcLoader.h>
+
+using android::hardware::graphics::composer::V2_1::IComposer;
+using android::hardware::graphics::composer::V2_1::passthrough::HwcLoader;
+
+extern "C" IComposer* HIDL_FETCH_IComposer(const char* /* name */) {
+    return HwcLoader::load();
+}