Add method to get current vsync from sf directly.

Bug: 205721584
Test: atest libsurfaceflinger_unittest
Test: atest DisplayEventReceiverTest
Change-Id: I38d4bd20bc2f2ad7ff964c3d613c28919478c0fc
diff --git a/services/surfaceflinger/tests/Android.bp b/services/surfaceflinger/tests/Android.bp
index 7b86229..2ede830 100644
--- a/services/surfaceflinger/tests/Android.bp
+++ b/services/surfaceflinger/tests/Android.bp
@@ -31,6 +31,7 @@
         "Credentials_test.cpp",
         "DereferenceSurfaceControl_test.cpp",
         "DisplayConfigs_test.cpp",
+        "DisplayEventReceiver_test.cpp",
         "EffectLayer_test.cpp",
         "InvalidHandles_test.cpp",
         "LayerCallback_test.cpp",