Add unit tests for android::compositionengine::impl::planner::Flattener

Fixes: 180672722
Test: adb shell /data/nativetest64/libcompositionengine_test/libcompositionengine_test
Change-Id: I9ac809cb94e7a1d33b0381a787360049aae4c459
diff --git a/services/surfaceflinger/CompositionEngine/Android.bp b/services/surfaceflinger/CompositionEngine/Android.bp
index 88998b0..1c47691 100644
--- a/services/surfaceflinger/CompositionEngine/Android.bp
+++ b/services/surfaceflinger/CompositionEngine/Android.bp
@@ -104,6 +104,7 @@
     defaults: ["libcompositionengine_defaults"],
     srcs: [
         "tests/planner/CachedSetTest.cpp",
+        "tests/planner/FlattenerTest.cpp",
         "tests/CompositionEngineTest.cpp",
         "tests/DisplayColorProfileTest.cpp",
         "tests/DisplayTest.cpp",