SF: hwvsync on for more samples on recalibration

Decrease the amount of time that the VSyncPredictor is using the
idealPeriod (ie, the hwc-reported period) by leaving hwVsync on until
there's enough info (4 additional pulses) to infer device's
measured/actual period.

This will benefit frames that come intermittently, as the vsync's
cumulative error over the larger time gap will be smaller, giving the
timer a better chance of landing closer to the vsync signal when those
one-off frames come in.

Test: 3 new unit tests, 2 other tests modified in substance.
Test: dogfooding
Test: uibench
Fixes: 159896971

Change-Id: Ic3190822e38c5a24d3445ea89ef6d7e34a590077