Record rendering stability histogram.

The new present2presentDelta histogram records the stability
in present2present timings between consecutive frames.

Bug: 239083860
Test: atest libsurfaceflinger_unittest
Change-Id: I9559daaab831c85ecb2aab0ef8ba0fdeb84bb345
diff --git a/services/surfaceflinger/TimeStats/TimeStats.h b/services/surfaceflinger/TimeStats/TimeStats.h
index 61d7c22..09c3217 100644
--- a/services/surfaceflinger/TimeStats/TimeStats.h
+++ b/services/surfaceflinger/TimeStats/TimeStats.h
@@ -219,6 +219,7 @@
         uint32_t lateAcquireFrames = 0;
         uint32_t badDesiredPresentFrames = 0;
         TimeRecord prevTimeRecord;
+        std::optional<int32_t> prevPresentToPresentMs;
         std::deque<TimeRecord> timeRecords;
     };