SurfaceFlinger: add frame rate to dumpsys

Print frame rate if explicitly set by setFrameRate to dumpsys

Bug: 155676501
Test: adb shell dumpsys SurfaceFlinger
Change-Id: I1016f8a56921f7f465ca50298f49d05b02be585c
diff --git a/services/surfaceflinger/Layer.h b/services/surfaceflinger/Layer.h
index 6636b5e..b630333 100644
--- a/services/surfaceflinger/Layer.h
+++ b/services/surfaceflinger/Layer.h
@@ -819,6 +819,7 @@
 
     bool setFrameRate(FrameRate frameRate);
     virtual FrameRate getFrameRateForLayerTree() const;
+    static std::string frameRateCompatibilityString(FrameRateCompatibility compatibility);
 
 protected:
     // constant