DMS: use vsync rate when calculating the app frame rate

This is a bug fix where without this CL, DMS does not report
the refresh rate correctly, for values that are not divisors of the
peak refresh rate, like 80hz.

Test: running swappy at 80hz
Test: presubmit (+ 1 new unit test)
Bug: 350082219
Flag: EXEMPT bugfix
Change-Id: I3c1d1832d97d931f12f5988bb6e17e25935335e2