FrameTimeline: Optimize tracing information about frame start time

If the prediction state is expired but the actual start time is
available, prioritize the actual start time as the reference for the
actual timeline instead of using the actual end time.

Flag: EXEMPT. bug fix from external contributor

Signed-off-by: liuzhifu <[email protected]>
(cherry picked from https://android-review.googlesource.com/q/commit:13eb5161b25da09feabfb9c4c89e6804da92056b)
Merged-In: I1646a159ab74b3d7918067e24c6e827bf6ae5b4e
Change-Id: I1646a159ab74b3d7918067e24c6e827bf6ae5b4e

Author: liuzhifu

services/surfaceflinger/FrameTimeline/FrameTimeline.cpp

@@ -801,6 +801,9 @@ void SurfaceFrame::traceActuals(int64_t displayFrameToken, nsecs_t monoBootOffse
     FrameTimelineDataSource::Trace([&](FrameTimelineDataSource::TraceContext ctx) {
         const auto timestamp = [&]() {
             std::scoped_lock lock(mMutex);
+            if (mActuals.startTime != 0) {
+                return mActuals.startTime;
+            }
             // Actual start time is not yet available, so use expected start instead
             if (mPredictionState == PredictionState::Expired) {
                 // If prediction is expired, we can't use the predicted start time. Instead, just
@@ -811,7 +814,7 @@ void SurfaceFrame::traceActuals(int64_t displayFrameToken, nsecs_t monoBootOffse
                 return endTime - kPredictionExpiredStartTimeDelta;
             }
 
-            return mActuals.startTime == 0 ? mPredictions.startTime : mActuals.startTime;
+            return mPredictions.startTime;
         }();
 
         if (filterFramesBeforeTraceStarts && !shouldTraceForDataSource(ctx, timestamp)) {