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.cpp b/services/surfaceflinger/DisplayHardware/HWComposer.cpp
index c4ea8cc..2d63bde 100644
--- a/services/surfaceflinger/DisplayHardware/HWComposer.cpp
+++ b/services/surfaceflinger/DisplayHardware/HWComposer.cpp
@@ -821,7 +821,7 @@
     return NO_ERROR;
 }
 
-sp<Fence> HWComposer::getLastRetireFence(int32_t id) {
+sp<Fence> HWComposer::getLastRetireFence(int32_t id) const {
     if (uint32_t(id)>31 || !mAllocatedDisplayIDs.hasBit(id))
         return Fence::NO_FENCE;
     return mDisplayData[id].lastRetireFence;