SF: Fix threshold for next VSYNC

If debug.sf.phase_offset_threshold_for_next_vsync_ns is not set, the
threshold is set to the maximum nsecs_t value. A narrowing conversion
caused the value to overflow to -1, leading to garbage phase offsets.

Bug: 138813900
Bug: 139284671
Bug: 138942712
Test: dumpsys SurfaceFlinger --vsync
Test: atest google/perf/app-transition/app-transition-cold
Change-Id: Ib7f732be95d16d29f4a71602b56795bee09be37b