SF: VSyncTracker, clear timestamps on beginResync

DispSync would clear its internal model around power events, as the
vsync timings during power mode changes are not accurate for screen-on
vsync prediction. DispSync uses the beginResync signal for this, and the
new system does the same (although once DispSync is deprecated,
begin/endResync should go away too).

Fixes: 146050690
Test: Dogfooding blueline
Test: 2 new unit tests
Change-Id: Ie4b607029516dcea0e5d65afca75a7017bf63e12
diff --git a/services/surfaceflinger/Scheduler/VSyncTracker.h b/services/surfaceflinger/Scheduler/VSyncTracker.h
index 6be63fe..2b27884 100644
--- a/services/surfaceflinger/Scheduler/VSyncTracker.h
+++ b/services/surfaceflinger/Scheduler/VSyncTracker.h
@@ -61,6 +61,9 @@
      */
     virtual void setPeriod(nsecs_t period) = 0;
 
+    /* Inform the tracker that the samples it has are not accurate for prediction. */
+    virtual void resetModel() = 0;
+
 protected:
     VSyncTracker(VSyncTracker const&) = delete;
     VSyncTracker& operator=(VSyncTracker const&) = delete;