SurfaceFlinger: refresh rate calculation when no timestamps

Frames without presentation timestamp should be excluded from refresh
rate calculation. This changes skips these frames instead of just
declaring refresh rate unknown. This impacts scenarios that we get few
frames without presentation timestamps where the majority does include
them.

Bug: 155710271
Test: Facebook feed videos
Change-Id: Id79f4d6cb87b2fd6fb787fab2d863f50be99cac5