SF: VSyncReactor add timestamp querying functions

Adds 2 more functions from the DispSync interface to VSyncReactor,
computeNextRefresh, and expectedPresentTime

Bug: 140303479
Test: 3 new units

Change-Id: I75ea79ed749f05daf3337ddc8ca145e2ecceabcd
diff --git a/services/surfaceflinger/Scheduler/VSyncPredictor.h b/services/surfaceflinger/Scheduler/VSyncPredictor.h
index 1590f49..4210b3c 100644
--- a/services/surfaceflinger/Scheduler/VSyncPredictor.h
+++ b/services/surfaceflinger/Scheduler/VSyncPredictor.h
@@ -39,6 +39,7 @@
 
     void addVsyncTimestamp(nsecs_t timestamp) final;
     nsecs_t nextAnticipatedVSyncTimeFrom(nsecs_t timePoint) const final;
+    nsecs_t currentPeriod() const final;
 
     /*
      * Inform the model that the period is anticipated to change to a new value.