Add sysprop for refresh rate threshold.

When the sysprop is enabled, layers asking for heuristic or multiple
will not vote for multiples >= $threshold amount. For example, 24 fps
would normally vote 120 hz because it is a multiple. Instead, it should
use the scoring calculation to determine a refresh rate < 120 Hz.

This addresses 24 fps videos for 60Hz and 120 Hz displays. The request
wants the refresh rate vote to be 60 Hz. (see bug)

BUG: 190815773
Test: atest libsurfaceflinger_tests
Change-Id: If2619258b93fb9c30e09253266d3d65e1a732047