SF Buffer Stuffing

When SF presents a frame because it started late (because the previous
frame ran too long), this case is similar to App Buffer Stuffing. The
frame rate will be smooth but all frames have an increased latency.
Currently, we mark this as SFScheduling jank. This change classifies
these frames as a separate jank called SFStuffing. This is useful in
weeding out issues related to frame rate vs latency.

Bug: 181593046
Test: libsurfaceflinger_unittest

Change-Id: I59a2931c5e636e9a38fd07e0a328d37873969c44