SF: Define mock::RE::RenderEngine et al

This allows the unit test to use mocks in place of the normal
implementation.

Test: Builds
Bug: None
Change-Id: Iad51c4cf2373ef4313fc9ac77d1df4aa15d705ea
diff --git a/services/surfaceflinger/tests/unittests/Android.bp b/services/surfaceflinger/tests/unittests/Android.bp
index 6151466..be3a918 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",
+        "MockRenderEngine.cpp",
     ],
     static_libs: [
         "libgmock",