Add logging when refresh rate policy is invalid

Add some logging so we can more easily debug situations
where the refrash rate policy is invalid.

Bug: 172637939
Test: presubmit
Change-Id: I7309f8aa93e4d02d5fecf65ab3b671037180d92c
diff --git a/services/surfaceflinger/Scheduler/RefreshRateConfigs.h b/services/surfaceflinger/Scheduler/RefreshRateConfigs.h
index 5cf7d07..8ff92a0 100644
--- a/services/surfaceflinger/Scheduler/RefreshRateConfigs.h
+++ b/services/surfaceflinger/Scheduler/RefreshRateConfigs.h
@@ -166,7 +166,7 @@
         }
 
         bool operator!=(const Policy& other) const { return !(*this == other); }
-        std::string toString();
+        std::string toString() const;
     };
 
     // Return code set*Policy() to indicate the current policy is unchanged.