SF: delay region sampling when short on time

In a number of janky traces, particularly at high
frame rates, we've seen the surfaceflinger thread
overrunning its time slot. In some of those cases,
the surfaceflinger thread is doing region-sampling.
This change causes region-sampling to check how much
time is left until the next vsync before deciding
whether to sample this frame. If low on time,
it will defer the sampling to a later frame.

Bug: 133779857
Test: trace inspection from scrolling in various apps
Change-Id: I92c2368e80033c1ba6e27f947a456d14db02064c