[sf] only update the last latch time if the layer has a buffer

If we apply a transaction that does not latch a buffer, we
incorrectly updated the last latch time. In the next vsync,
if we presented a buffer late, we would misclassify the
jank as buffer stuffing.

Fix this by only updating the latch time if the layer has
a buffer.

Test: verify in perfetto traces
Fixes: 270041584
Change-Id: I423a28803a2660ada80ef554469f3977c389fbfa