Improve SurfaceFlinger PTS estimation

Get the next refresh time from DispSync instead of guessing based
on the current time.

Change-Id: I8dc72a3217bfd4e9b4c905034194d1a298cad69a
diff --git a/services/surfaceflinger/DisplayHardware/HWComposer.h b/services/surfaceflinger/DisplayHardware/HWComposer.h
index 9218bf6..e93bb7e 100644
--- a/services/surfaceflinger/DisplayHardware/HWComposer.h
+++ b/services/surfaceflinger/DisplayHardware/HWComposer.h
@@ -142,7 +142,7 @@
     // signal when the h/w composer is completely finished with the frame.
     // For physical displays, it is no longer being displayed. For virtual
     // displays, writes to the output buffer are complete.
-    sp<Fence> getLastRetireFence(int32_t id);
+    sp<Fence> getLastRetireFence(int32_t id) const;
 
     /*
      * Interface to hardware composer's layers functionality.