SF: VSyncReactor add period modification functions

Adds 2 more functions from the DispSync interface to VSyncReactor,
{get,set}Period

Bug: 140303479
Test: 2 new units

Change-Id: Ie0084597c80d6d43099201fb49b71ae9badea3ee
diff --git a/services/surfaceflinger/Scheduler/VSyncReactor.h b/services/surfaceflinger/Scheduler/VSyncReactor.h
index 8436e8a..73a09f1 100644
--- a/services/surfaceflinger/Scheduler/VSyncReactor.h
+++ b/services/surfaceflinger/Scheduler/VSyncReactor.h
@@ -40,6 +40,9 @@
     nsecs_t computeNextRefresh(int periodOffset) const;
     nsecs_t expectedPresentTime();
 
+    void setPeriod(nsecs_t period);
+    nsecs_t getPeriod();
+
 private:
     std::unique_ptr<Clock> const mClock;
     std::unique_ptr<VSyncDispatch> const mDispatch;