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/LayerHistory.h b/services/surfaceflinger/Scheduler/LayerHistory.h
index 507ccc6..4214bab 100644
--- a/services/surfaceflinger/Scheduler/LayerHistory.h
+++ b/services/surfaceflinger/Scheduler/LayerHistory.h
@@ -46,7 +46,7 @@
     ~LayerHistory();
 
     // Layers are unregistered when the weak reference expires.
-    void registerLayer(Layer*, float lowRefreshRate, float highRefreshRate, LayerVoteType type);
+    void registerLayer(Layer*, Fps highRefreshRate, LayerVoteType type);
 
     // Sets the display size. Client is responsible for synchronization.
     void setDisplayArea(uint32_t displayArea) { mDisplayArea = displayArea; }