SurfaceFlinger: store fps instead of duration in LayerInfo

To get a better average of the content fps, we switch to store the
momentarily fps rate instead of the refresh duration.
For example:

Frame#0 at 0ms
Frame#1 at 100ms
Frame#2 at 111ms
Frame#3 at 122ms
Average based on duration is AVERAGE(100, 11, 11) = 40.6ms (25fps)
Average based on fps is AVERAGE(10, 90, 90) = 63fps

Test: app launch
Bug: 136558136
Change-Id: Icab848dd1f312498590f9735b8881ecdf0d24113
diff --git a/services/surfaceflinger/Scheduler/LayerInfo.cpp b/services/surfaceflinger/Scheduler/LayerInfo.cpp
index beddb9b..e782dd5 100644
--- a/services/surfaceflinger/Scheduler/LayerInfo.cpp
+++ b/services/surfaceflinger/Scheduler/LayerInfo.cpp
@@ -50,7 +50,8 @@
     // Ignore time diff that are too high - those are stale values
     if (timeDiff > OBSOLETE_TIME_EPSILON_NS.count()) return;
     const nsecs_t refreshDuration = (timeDiff > 0) ? timeDiff : mMinRefreshDuration;
-    mRefreshRateHistory.insertRefreshRate(refreshDuration);
+    const int fps = 1e9f / refreshDuration;
+    mRefreshRateHistory.insertRefreshRate(fps);
 }
 
 } // namespace scheduler