Change swap chain stuffed detection logic

bug:29771461
bug:29413700
bug:30181577

Changes frame interval gap detection to look for wider gaps, as they
were incorrectly firing all the time.

Also adds a 500ms minimum gap between frames dropped because of stuffed
swap chain, to prevent dropping too often.

Change-Id: If16ed637d54bf37015704be102c5c2e3731a0824