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.cpp b/services/surfaceflinger/Scheduler/DispSyncSource.cpp
index 776e984..4e3f85f 100644
--- a/services/surfaceflinger/Scheduler/DispSyncSource.cpp
+++ b/services/surfaceflinger/Scheduler/DispSyncSource.cpp
@@ -30,6 +30,7 @@
 #include "EventThread.h"
 
 namespace android {
+using base::StringAppendF;
 
 DispSyncSource::DispSyncSource(DispSync* dispSync, nsecs_t phaseOffset, bool traceVsync,
                                const char* name)
@@ -107,6 +108,12 @@
     }
 }
 
+void DispSyncSource::dump(std::string& result) const {
+    std::lock_guard lock(mVsyncMutex);
+    StringAppendF(&result, "DispSyncSource: %s(%s)\n", mName, mEnabled ? "enabled" : "disabled");
+    mDispSync->dump(result);
+}
+
 } // namespace android
 
 // TODO(b/129481165): remove the #pragma below and fix conversion issues