SF: Define mock::EventThread

This allows the unit tests to use a Mock for the normal EventThread
instance.

Test: Builds
Bug: None
Change-Id: I6f6853a33d0cd9af4548eb318b4564eb6c9af1e6
diff --git a/services/surfaceflinger/tests/unittests/Android.bp b/services/surfaceflinger/tests/unittests/Android.bp
index be3a918..3eb07ae 100644
--- a/services/surfaceflinger/tests/unittests/Android.bp
+++ b/services/surfaceflinger/tests/unittests/Android.bp
@@ -21,6 +21,7 @@
         ":libsurfaceflinger_sources",
         "DisplayTransactionTest.cpp",
         "MockComposer.cpp",
+        "MockEventThread.cpp",
         "MockRenderEngine.cpp",
     ],
     static_libs: [