SF: Adding Mock Display and tests for RefreshRateStats.

Following ag/6005545.

Test: adding SF tests.
Bug: 113612090
Change-Id: Ic4d1ca622f00c4396ae19c44defac0c69fabed1d
diff --git a/services/surfaceflinger/SurfaceFlinger.cpp b/services/surfaceflinger/SurfaceFlinger.cpp
index 8746a68..e298e20 100644
--- a/services/surfaceflinger/SurfaceFlinger.cpp
+++ b/services/surfaceflinger/SurfaceFlinger.cpp
@@ -4338,7 +4338,7 @@
 
     if (display->isPrimary()) {
         mTimeStats->setPowerMode(mode);
-        if (mUseScheduler) {
+        if (mUseScheduler && mRefreshRateStats) {
             // Update refresh rate stats.
             mRefreshRateStats->setPowerMode(mode);
         }