SF: Setup CompositionEngine::Layer

Add a Layer class to CompositionEngine, and modify ColorLayer and
BufferLayer to create them.

Also sets up CompositionEngine::LayerFE as an interface to be
implemented by the SF Layer class.

Test: atest libsurfaceflinger_unittest libcompositionengine_test
Bug: 121291683
Change-Id: I2ed83355e81f36954b034ec60dd1a24c631f4874
diff --git a/services/surfaceflinger/CompositionEngine/mock/Layer.cpp b/services/surfaceflinger/CompositionEngine/mock/Layer.cpp
new file mode 100644
index 0000000..08483cb
--- /dev/null
+++ b/services/surfaceflinger/CompositionEngine/mock/Layer.cpp
@@ -0,0 +1,26 @@
+/*
+ * Copyright 2019 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 <compositionengine/mock/Layer.h>
+
+namespace android::compositionengine::mock {
+
+// The Google Mock documentation recommends explicit non-header instantiations
+// for better compile time performance.
+Layer::Layer() = default;
+Layer::~Layer() = default;
+
+} // namespace android::compositionengine::mock