Return janktype properly for dropped frames

In SurfaceFrame::getJankType(), a dropped frame won't have any present
time set and will return std::nullopt currently. This behavior can
result in a memory leak in releasePendingBuffer. Whenever SurfaceFrames
are created, they are added to a deque 'mPendingJankClassification'.
This deque is popped from the front as and when the frames are
presented. However, if a dropped frame is in the front of the deque, the
deque keeps growing in size to infinity. Since getJankType() reports a
std::nullopt, it assumes that the frame hasn't been presented yet and
keeps waiting for it.

Bug: 179701581
Test: libsurfaceflinger_unittest
Change-Id: I8ad1ed07222d39e4f98e4f7f3178db9fe52ea712