Dynamic framerate based on timestamps can become zero when input frames
are supplied at >1 sec intervals.

Bug: 228828942
Test: android.media.cts.EncodeVirtualDisplayWithCompositionTest#testVirtualDisplayRecycles
Test: com.google.android.media.gts.RtcVideoCodecTest#testDynamicFramerateChangeVp8
Change-Id: I3ceefaad9b420f86e4f0b524e6f0b5649af9997b