Support SurfaceView synchronization.

Add API for fetching the next frame number to be produced by
a given buffer producer. Add an API to SurfaceComposer to 
defer execution of the current transaction until a given frame number. 
Together these may be used to synchronize app drawing and surface 
control updates.

Change-Id: I8e0f4993332ac0199c768c88581a453fefbaff1d
diff --git a/services/surfaceflinger/DisplayHardware/VirtualDisplaySurface.h b/services/surfaceflinger/DisplayHardware/VirtualDisplaySurface.h
index d53d43c..fb37373 100644
--- a/services/surfaceflinger/DisplayHardware/VirtualDisplaySurface.h
+++ b/services/surfaceflinger/DisplayHardware/VirtualDisplaySurface.h
@@ -119,6 +119,7 @@
     virtual status_t allowAllocation(bool allow);
     virtual status_t setGenerationNumber(uint32_t generationNumber);
     virtual String8 getConsumerName() const override;
+    virtual uint64_t getNextFrameNumber() const override;
 
     //
     // Utility methods