SurfaceFlinger: add debug information when vsync is not generated

Print debug information about VsyncReactor internal state when
we expect a vsync event but it is not generated.

Bug: 151892277
Test: adb shell dumpsys SurfaceFlinger --dispsync
Test: simulate a missed vsync and observed logcat
Change-Id: Ia98cb884321d0c25610c4d721f422d9cd05c8c5e
diff --git a/services/surfaceflinger/Scheduler/VSyncPredictor.h b/services/surfaceflinger/Scheduler/VSyncPredictor.h
index ef1d88a..3ca878d 100644
--- a/services/surfaceflinger/Scheduler/VSyncPredictor.h
+++ b/services/surfaceflinger/Scheduler/VSyncPredictor.h
@@ -60,6 +60,8 @@
 
     std::tuple<nsecs_t /* slope */, nsecs_t /* intercept */> getVSyncPredictionModel() const;
 
+    void dump(std::string& result) const final;
+
 private:
     VSyncPredictor(VSyncPredictor const&) = delete;
     VSyncPredictor& operator=(VSyncPredictor const&) = delete;