gui: add test for region sampling

Adds a test that checks the correct luma is produced under a few
scenarios with different layerings.

Bug: 119639245
Test: ./libgui_test --gtest_filter="RegionSamplingTest.*"
Change-Id: Ic85a97f475a3414a79d3719bbd0b2b648bbccfb0
diff --git a/libs/gui/tests/Android.bp b/libs/gui/tests/Android.bp
index a5b87ec..ab6dcaa 100644
--- a/libs/gui/tests/Android.bp
+++ b/libs/gui/tests/Android.bp
@@ -23,6 +23,7 @@
         "IGraphicBufferProducer_test.cpp",
         "Malicious.cpp",
         "MultiTextureConsumer_test.cpp",
+        "RegionSampling_test.cpp",
         "StreamSplitter_test.cpp",
         "SurfaceTextureClient_test.cpp",
         "SurfaceTextureFBO_test.cpp",