Add Fps class

Add a class to wrap a fps value. This is useful because across
the code we
 - convert between vsyncPeriod and fps
 - compare with tolerance (this ensures we use consistent tolerance)
 - consistent toString method

Bug: 159590486
Test: presubmit
Change-Id: Iebb77a33a2f822056642aa61bd6fac6514aa656d
diff --git a/services/surfaceflinger/Scheduler/RefreshRateStats.h b/services/surfaceflinger/Scheduler/RefreshRateStats.h
index d9e7b37..ce91a76 100644
--- a/services/surfaceflinger/Scheduler/RefreshRateStats.h
+++ b/services/surfaceflinger/Scheduler/RefreshRateStats.h
@@ -115,8 +115,10 @@
                 mConfigModesTotalTime[mCurrentConfigMode] = 0;
             }
             mConfigModesTotalTime[mCurrentConfigMode] += timeElapsedMs;
-            fps = static_cast<uint32_t>(std::round(
-                    mRefreshRateConfigs.getRefreshRateFromConfigId(mCurrentConfigMode).getFps()));
+            fps = static_cast<uint32_t>(
+                    std::round(mRefreshRateConfigs.getRefreshRateFromConfigId(mCurrentConfigMode)
+                                       .getFps()
+                                       .getValue()));
         } else {
             mScreenOffTime += timeElapsedMs;
         }