libs/ui: Add MockFence

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

Change-Id: I37cf6ce77cb446179b73dfc21f79b19eba2be58f
diff --git a/libs/ui/Android.bp b/libs/ui/Android.bp
index ba5a64f..04ba485 100644
--- a/libs/ui/Android.bp
+++ b/libs/ui/Android.bp
@@ -86,6 +86,7 @@
 
     export_include_dirs: [
         "include",
+        "include_mock",
         "include_private",
         "include_types",
     ],