Add unit test for boot display mode

Bug: 213909104
Test: atest SurfaceFlinger_test
Change-Id: I68a578ede77c66c85278190e34225d95d26d52e8
diff --git a/services/surfaceflinger/tests/Android.bp b/services/surfaceflinger/tests/Android.bp
index 7b86229..88a954e 100644
--- a/services/surfaceflinger/tests/Android.bp
+++ b/services/surfaceflinger/tests/Android.bp
@@ -26,6 +26,7 @@
     defaults: ["surfaceflinger_defaults"],
     test_suites: ["device-tests"],
     srcs: [
+        "BootDisplayMode_test.cpp",
         "BufferGenerator.cpp",
         "CommonTypes_test.cpp",
         "Credentials_test.cpp",