Consolidate PostWorker::compose() interface

... by converting `ComposeDevice` and `ComposeDevice_v2` into a
single `FlatComposeRequest` and having `PostWorker` use this single
flattened type. Also, make some const correctness changes that
touch the compose request.

Bug: b/229145718
Test: `launch_cvd --gpu_mode=gfxstream`
Change-Id: I0eb9ae7a4ff9a91e9f47a1460def86967034779b
diff --git a/stream-servers/Hwc2.cpp b/stream-servers/Hwc2.cpp
new file mode 100644
index 0000000..bf82ef1
--- /dev/null
+++ b/stream-servers/Hwc2.cpp
@@ -0,0 +1,37 @@
+// Copyright 2022 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 expresso or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "Hwc2.h"
+
+std::unique_ptr<FlatComposeRequest> ToFlatComposeRequest(
+        const ComposeDevice* composeRequest) {
+    auto flatComposeRequest = std::make_unique<FlatComposeRequest>();
+    flatComposeRequest->displayId = 0;
+    flatComposeRequest->targetHandle = composeRequest->targetHandle;
+    for (uint32_t i = 0; i < composeRequest->numLayers; i++) {
+        flatComposeRequest->layers.emplace_back(composeRequest->layer[i]);
+    }
+    return flatComposeRequest;
+}
+
+std::unique_ptr<FlatComposeRequest> ToFlatComposeRequest(
+        const ComposeDevice_v2* composeRequest) {
+    auto flatComposeRequest = std::make_unique<FlatComposeRequest>();
+    flatComposeRequest->displayId = composeRequest->displayId;
+    flatComposeRequest->targetHandle = composeRequest->targetHandle;
+    for (uint32_t i = 0; i < composeRequest->numLayers; i++) {
+        flatComposeRequest->layers.emplace_back(composeRequest->layer[i]);
+    }
+    return flatComposeRequest;
+}