Exclude BufferStuffing from TimeStats atoms

Since BufferStuffing is not really a jank, including this in the jank
reasons makes it hard to find genuine number of janky frames due to
server side shortcomings.

Bug: 185595205
Test: libsurfaceflinger_unittest
Change-Id: I5e355f883c374a27b0e44453cfb857f059eb23d0