SF TimeStats: fix missed frame logic

The current missed frames count is correct, but the decrement on the
total frames when mPropagateBackpressure is enabled is not right since
the missed frame won't count twice on SurfaceFlinger side. This change
fixed this logic error.

Test: dumpsys SurfaceFlinger --timestats <see go/sf-timestats for args>
Bug: b/70388650
Change-Id: I3bab221ad4b9f44da965084fd52da6c82523d1f4