Fix ts for prediction expired dropped frames

We have a gap in our logic where dropped frames that have their
predictions expired will send out incorrect timestamps. This has caused
some instrumentation failures at the perfetto side.

Bug: 185978397
Test: libsurfaceflinger_unittest
Change-Id: I301816893f425301a5ee1065685b9f7007d8cf1d
diff --git a/services/surfaceflinger/FrameTimeline/FrameTimeline.cpp b/services/surfaceflinger/FrameTimeline/FrameTimeline.cpp
index 7468ac3..be552c6 100644
--- a/services/surfaceflinger/FrameTimeline/FrameTimeline.cpp
+++ b/services/surfaceflinger/FrameTimeline/FrameTimeline.cpp
@@ -653,8 +653,10 @@
             // If prediction is expired, we can't use the predicted start time. Instead, just use a
             // start time a little earlier than the end time so that we have some info about this
             // frame in the trace.
+            nsecs_t endTime =
+                    (mPresentState == PresentState::Dropped ? mDropTime : mActuals.endTime);
             packet->set_timestamp(
-                    static_cast<uint64_t>(mActuals.endTime - kPredictionExpiredStartTimeDelta));
+                    static_cast<uint64_t>(endTime - kPredictionExpiredStartTimeDelta));
         } else {
             packet->set_timestamp(static_cast<uint64_t>(mPredictions.startTime));
         }