SF: Define Hwc2::mock::Composer

This allows the unit test to use a mock in place of android::Hwc2::Composer.

Test: Code builds
Bug: None
Change-Id: Ia6f0df083d579cf69e97abc2ff7f2d3f5789a320
diff --git a/services/surfaceflinger/tests/unittests/Android.bp b/services/surfaceflinger/tests/unittests/Android.bp
index 72e0a0f..6151466 100644
--- a/services/surfaceflinger/tests/unittests/Android.bp
+++ b/services/surfaceflinger/tests/unittests/Android.bp
@@ -20,6 +20,7 @@
     srcs: [
         ":libsurfaceflinger_sources",
         "DisplayTransactionTest.cpp",
+        "MockComposer.cpp",
     ],
     static_libs: [
         "libgmock",