libs/ui: Add MockFence

Add a MockFence class to allow unit tests to mock fences.

Change-Id: I37cf6ce77cb446179b73dfc21f79b19eba2be58f
diff --git a/libs/ui/tests/Android.bp b/libs/ui/tests/Android.bp
index 516aad8..a87740a 100644
--- a/libs/ui/tests/Android.bp
+++ b/libs/ui/tests/Android.bp
@@ -115,3 +115,12 @@
     srcs: ["Size_test.cpp"],
     cflags: ["-Wall", "-Werror"],
 }
+
+cc_test {
+    name: "MockFence_test",
+    shared_libs: ["libui"],
+    static_libs: ["libgmock"],
+    srcs: ["MockFence_test.cpp"],
+    cflags: ["-Wall", "-Werror"],
+}
+