SurfaceFlinger: fix a crash with RefreshRateOverlay

 - Fix a nullptr deference in createLayer
 - Add a test for RefreshRateOverlay

Test: adb shell /data/nativetest64/SurfaceFlinger_test/SurfaceFlinger_test
Bug: 169867983
Change-Id: I496aaeceaa86b55f806ddc921fc5b050417725b7
diff --git a/TEST_MAPPING b/TEST_MAPPING
index 5db44c7..307e21c 100644
--- a/TEST_MAPPING
+++ b/TEST_MAPPING
@@ -53,6 +53,9 @@
         },
         {
           "include-filter": "*RelativeZTest.*"
+        },
+        {
+          "include-filter": "*RefreshRateOverlayTest.*"
         }
       ]
     },