SF: Flag guard setFrameRate() API

- LayerHistory is now always created
- Do not use LayerHistory when content detection flag is turned off.

Bug: 148428554
Test: atest CtsGraphicsTestCases:SetFrameRateTest
Test: Play YouTube video. When content detection flag is off,
      display runs at 90, otherwise at 60.
Test: adb shell /data/.../libsurfaceflinger_unittest
Change-Id: Iab8b79e16a5257c770d9ddd204cfe26d22de83b6
diff --git a/services/surfaceflinger/SurfaceFlinger.h b/services/surfaceflinger/SurfaceFlinger.h
index ccf5794..8cabcf0 100644
--- a/services/surfaceflinger/SurfaceFlinger.h
+++ b/services/surfaceflinger/SurfaceFlinger.h
@@ -248,6 +248,11 @@
     static ui::Dataspace wideColorGamutCompositionDataspace;
     static ui::PixelFormat wideColorGamutCompositionPixelFormat;
 
+    // Whether to use frame rate API when deciding about the refresh rate of the display. This
+    // variable is caches in SF, so that we can check it with each layer creation, and a void the
+    // overhead that is caused by reading from sysprop.
+    static bool useFrameRateApi;
+
     static char const* getServiceName() ANDROID_API {
         return "SurfaceFlinger";
     }