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/DispSyncSource.h b/services/surfaceflinger/Scheduler/DispSyncSource.h
index 328b8c1..f278712 100644
--- a/services/surfaceflinger/Scheduler/DispSyncSource.h
+++ b/services/surfaceflinger/Scheduler/DispSyncSource.h
@@ -36,6 +36,8 @@
     void setCallback(VSyncSource::Callback* callback) override;
     void setPhaseOffset(nsecs_t phaseOffset) override;
 
+    void dump(std::string&) const override;
+
 private:
     // The following method is the implementation of the DispSync::Callback.
     virtual void onDispSyncEvent(nsecs_t when);
@@ -52,7 +54,7 @@
     std::mutex mCallbackMutex;
     VSyncSource::Callback* mCallback GUARDED_BY(mCallbackMutex) = nullptr;
 
-    std::mutex mVsyncMutex;
+    mutable std::mutex mVsyncMutex;
     TracedOrdinal<nsecs_t> mPhaseOffset GUARDED_BY(mVsyncMutex);
     bool mEnabled GUARDED_BY(mVsyncMutex) = false;
 };