[SurfaceFlinger] Exclude first vsync duration from period calculation

Some displays don't exit from idle state immediately when vsync is
enabled through the kernel idle timer, so exclude the first vsync period
instead of just the first sample.

Bug: 136197211
Test: Scrolling through settings
Change-Id: I899fec289c8b08cfaa3264f7f6291a6fac489ab8
(cherry picked from commit 94390a32)
Merged-In: I899fec289c8b08cfaa3264f7f6291a6fac489ab8